目录
参考资料
- JDK 8 官方文档 - https://docs.oracle.com/javase/8/
- Java HotSpot VM Options （JDK 7 或 7 以前版本）- https://www.oracle.com/java/technologies/javase/vmoptions-jsp.html
- Java Tool Reference （Window） - https://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html
- Java Tool Reference （Unix）- https://docs.oracle.com/javase/8/docs/technotes/tools/unix/index.html
1 JVM 选项
java命令行工具执行时可添加 Java HotSpot VM Options。
使用方式：java -XX:+HeapDumpOnOutOfMemoryError -jar demo.jar
1.1 生产常用 JVM 选项
JDK 1.7
以下参数主要用于 JVM 进程崩溃排查问题使用，指定了错误文件和内存溢出 dump 文件地址。
-Xms256m
-Xmx1024m
-XX:ErrorFile=./hs_err_pid<pid>.log
-XX:HeapDumpPath=./java_pid<pid>.hprof
-XX:+HeapDumpOnOutOfMemoryError
-XX:+PrintCommandLineFlags
-XX:+PrintGC
-XX:+PrintGCDetails
-XX:+PrintTenuringDistribution
-XX:+PrintGCDateStamps
-XX:+PrintGCTimeStamps
1.2 JDK 1.7 (常用 -XX 选项)
https://www.oracle.com/java/technologies/javase/vmoptions-jsp.html
-XX 选项类型及赋值规则：
- boolean 类型 -
-XX:+<option>表示开启,-XX:-<option>关闭 - Number 类型-
-XX:<option>=<number> - String 类型 -
-XX:<option>=<string>
（1）行为选项
| Option and Default Value | Description |
|---|---|
| -XX:-AllowUserSignalHandlers | Do not complain if the application installs signal handlers. (Relevant to Solaris and Linux only.) |
| -XX:AltStackSize=16384 | Alternate signal stack size (in Kbytes). (Relevant to Solaris only, removed from 5.0.) |
| -XX:-DisableExplicitGC | By default calls to System.gc() are enabled (-XX:-DisableExplicitGC). Use -XX:+DisableExplicitGC to disable calls to System.gc(). Note that the JVM still performs garbage collection when necessary. |
| -XX:+FailOverToOldVerifier | Fail over to old verifier when the new type checker fails. (Introduced in 6.) |
| -XX:+HandlePromotionFailure | The youngest generation collection does not require a guarantee of full promotion of all live objects. (Introduced in 1.4.2 update 11) [5.0 and earlier: false.] |
| -XX:+MaxFDLimit | Bump the number of file descriptors to max. (Relevant to Solaris only.) |
| -XX:PreBlockSpin=10 | Spin count variable for use with -XX:+UseSpinning. Controls the maximum spin iterations allowed before entering operating system thread synchronization code. (Introduced in 1.4.2.) |
| -XX:-RelaxAccessControlCheck | Relax the access control checks in the verifier. (Introduced in 6.) |
| -XX:+ScavengeBeforeFullGC | Do young generation GC prior to a full GC. (Introduced in 1.4.1.) |
| -XX:+UseAltSigs | Use alternate signals instead of SIGUSR1 and SIGUSR2 for VM internal signals. (Introduced in 1.3.1 update 9, 1.4.1. Relevant to Solaris only.) |
| -XX:+UseBoundThreads | Bind user level threads to kernel threads. (Relevant to Solaris only.) |
| -XX:-UseConcMarkSweepGC | Use concurrent mark-sweep collection for the old generation. (Introduced in 1.4.1) |
| -XX:+UseGCOverheadLimit | Use a policy that limits the proportion of the VM's time that is spent in GC before an OutOfMemory error is thrown. (Introduced in 6.) |
| -XX:+UseLWPSynchronization | Use LWP-based instead of thread based synchronization. (Introduced in 1.4.0. Relevant to Solaris only.) |
| -XX:-UseParallelGC | Use parallel garbage collection for scavenges. (Introduced in 1.4.1) |
| -XX:-UseParallelOldGC | Use parallel garbage collection for the full collections. Enabling this option automatically sets -XX:+UseParallelGC. (Introduced in 5.0 update 6.) |
| -XX:-UseSerialGC | Use serial garbage collection. (Introduced in 5.0.) |
| -XX:-UseSpinning | Enable naive spinning on Java monitor before entering operating system thread synchronizaton code. (Relevant to 1.4.2 and 5.0 only.) [1.4.2, multi-processor Windows platforms: true] |
| -XX:+UseTLAB | Use thread-local object allocation (Introduced in 1.4.0, known as UseTLE prior to that.) [1.4.2 and earlier, x86 or with -client: false] |
| -XX:+UseSplitVerifier | Use the new type checker with StackMapTable attributes. (Introduced in 5.0.)[5.0: false] |
| -XX:+UseThreadPriorities | Use native thread priorities. |
| -XX:+UseVMInterruptibleIO | Thread interrupt before or with EINTR for I/O operations results in OS_INTRPT. (Introduced in 6. Relevant to Solaris only.) |
| （2）G1 垃圾回收器选项 |
| Option and Default Value | Description |
|---|---|
| -XX:+UseG1GC | Use the Garbage First (G1) Collector |
| -XX:MaxGCPauseMillis=n | Sets a target for the maximum GC pause time. This is a soft goal, and the JVM will make its best effort to achieve it. |
| -XX:InitiatingHeapOccupancyPercent=n | Percentage of the (entire) heap occupancy to start a concurrent GC cycle. It is used by GCs that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (e.g., G1). A value of 0 denotes 'do constant GC cycles'. The default value is 45. |
| -XX:NewRatio=n | Ratio of old/new generation sizes. The default value is 2. |
| -XX:SurvivorRatio=n | Ratio of eden/survivor space size. The default value is 8. |
| -XX:MaxTenuringThreshold=n | Maximum value for tenuring threshold. The default value is 15. |
| -XX:ParallelGCThreads=n | Sets the number of threads used during parallel phases of the garbage collectors. The default value varies with the platform on which the JVM is running. |
| -XX:ConcGCThreads=n | Number of threads concurrent garbage collectors will use. The default value varies with the platform on which the JVM is running. |
| -XX:G1ReservePercent=n | Sets the amount of heap that is reserved as a false ceiling to reduce the possibility of promotion failure. The default value is 10. |
| -XX:G1HeapRegionSize=n | With G1 the Java heap is subdivided into uniformly sized regions. This sets the size of the individual sub-divisions. The default value of this parameter is determined ergonomically based upon heap size. The minimum value is 1Mb and the maximum value is 32Mb. |
（3）性能调优选项
| Option and Default Value | Description |
|---|---|
| -XX:+AggressiveOpts | Turn on point performance compiler optimizations that are expected to be default in upcoming releases. (Introduced in 5.0 update 6.) |
| -XX:CompileThreshold=10000 | Number of method invocations/branches before compiling [-client: 1,500] |
| -XX:LargePageSizeInBytes=4m | Sets the large page size used for the Java heap. (Introduced in 1.4.0 update 1.) [amd64: 2m.] |
| -XX:MaxHeapFreeRatio=70 | Maximum percentage of heap free after GC to avoid shrinking. |
| -XX:MaxNewSize=size | Maximum size of new generation (in bytes). Since 1.4, MaxNewSize is computed as a function of NewRatio. [1.3.1 Sparc: 32m; 1.3.1 x86: 2.5m.] |
| -XX:MaxPermSize=64m | Size of the Permanent Generation. [5.0 and newer: 64 bit VMs are scaled 30% larger; 1.4 amd64: 96m; 1.3.1 -client: 32m.] |
| -XX:MinHeapFreeRatio=40 | Minimum percentage of heap free after GC to avoid expansion. |
| -XX:NewRatio=2 | Ratio of old/new generation sizes. [Sparc -client: 8; x86 -server: 8; x86 -client: 12.]-client: 4 (1.3) 8 (1.3.1+), x86: 12] |
| -XX:NewSize=2m | Default size of new generation (in bytes) [5.0 and newer: 64 bit VMs are scaled 30% larger; x86: 1m; x86, 5.0 and older: 640k] |
| -XX:ReservedCodeCacheSize=32m | Reserved code cache size (in bytes) - maximum code cache size. [Solaris 64-bit, amd64, and -server x86: 2048m; in 1.5.0_06 and earlier, Solaris 64-bit and amd64: 1024m.] |
| -XX:SurvivorRatio=8 | Ratio of eden/survivor space size [Solaris amd64: 6; Sparc in 1.3.1: 25; other Solaris platforms in 5.0 and earlier: 32] |
| -XX:TargetSurvivorRatio=50 | Desired percentage of survivor space used after scavenge. |
| -XX:ThreadStackSize=512 | Thread Stack Size (in Kbytes). (0 means use default stack size) [Sparc: 512; Solaris x86: 320 (was 256 prior in 5.0 and earlier); Sparc 64 bit: 1024; Linux amd64: 1024 (was 0 in 5.0 and earlier); all others 0.] |
| -XX:+UseBiasedLocking | Enable biased locking. For more details, see this tuning example. (Introduced in 5.0 update 6.) [5.0: false] |
| -XX:+UseFastAccessorMethods | Use optimized versions of Get |
| -XX:-UseISM | Use Intimate Shared Memory. [Not accepted for non-Solaris platforms.] |
| -XX:+UseLargePages | Use large page memory. (Introduced in 5.0 update 5.) For details, see Java Support for Large Memory Pages. |
| -XX:+UseMPSS | Use Multiple Page Size Support w/4mb pages for the heap. Do not use with ISM as this replaces the need for ISM. (Introduced in 1.4.0 update 1, Relevant to Solaris 9 and newer.) [1.4.1 and earlier: false] |
| -XX:+UseStringCache | Enables caching of commonly allocated strings. |
| -XX:AllocatePrefetchLines=1 | Number of cache lines to load after the last object allocation using prefetch instructions generated in JIT compiled code. Default values are 1 if the last allocated object was an instance and 3 if it was an array. |
| -XX:AllocatePrefetchStyle=1 | Generated code style for prefetch instructions. 0 - no prefetch instructions are generated, 1 - execute prefetch instructions after each allocation, 2 - use TLAB allocation watermark pointer to gate when prefetch instructions are executed. |
| -XX:+UseCompressedStrings | Use a byte[] for Strings which can be represented as pure ASCII. (Introduced in Java 6 Update 21 Performance Release) |
| -XX:+OptimizeStringConcat | Optimize String concatenation operations where possible. (Introduced in Java 6 Update 20) |
（3）调试选项 - 定位问题
| Option and Default Value | Description |
|---|---|
| -XX:-CITime | Prints time spent in JIT Compiler. (Introduced in 1.4.0.) |
| -XX:ErrorFile=./hs_err_pid |
If an error occurs, save the error data to this file. (Introduced in 6.) |
| -XX:-ExtendedDTraceProbes | Enable performance-impacting dtrace probes. (Introduced in 6. Relevant to Solaris only.) |
| -XX:HeapDumpPath=./java_pid |
Path to directory or filename for heap dump. Manageable. (Introduced in 1.4.2 update 12, 5.0 update 7.) |
| -XX:-HeapDumpOnOutOfMemoryError | Dump heap to file when java.lang.OutOfMemoryError is thrown. Manageable. (Introduced in 1.4.2 update 12, 5.0 update 7.) |
| -XX:OnError=" |
Run user-defined commands on fatal error. (Introduced in 1.4.2 update 9.) |
| -XX:OnOutOfMemoryError=" |
Run user-defined commands when an OutOfMemoryError is first thrown. (Introduced in 1.4.2 update 12, 6) |
| -XX:-PrintClassHistogram | Print a histogram of class instances on Ctrl-Break. Manageable. (Introduced in 1.4.2.) The jmap -histo command provides equivalent functionality. |
| -XX:-PrintConcurrentLocks | Print java.util.concurrent locks in Ctrl-Break thread dump. Manageable. (Introduced in 6.) The jstack -l command provides equivalent functionality. |
| -XX:-PrintCommandLineFlags | Print flags that appeared on the command line. (Introduced in 5.0.) |
| -XX:-PrintCompilation | Print message when a method is compiled. |
| -XX:-PrintGC | Print messages at garbage collection. Manageable. |
| -XX:-PrintGCDetails | Print more details at garbage collection. Manageable. (Introduced in 1.4.0.) |
| -XX:-PrintGCTimeStamps | Print timestamps at garbage collection. Manageable (Introduced in 1.4.0.) |
| -XX:-PrintTenuringDistribution | Print tenuring age information. |
| -XX:-PrintAdaptiveSizePolicy | Enables printing of information about adaptive generation sizing. |
| -XX:-TraceClassLoading | Trace loading of classes. |
| -XX:-TraceClassLoadingPreorder | Trace all classes loaded in order referenced (not loaded). (Introduced in 1.4.2.) |
| -XX:-TraceClassResolution | Trace constant pool resolutions. (Introduced in 1.4.2.) |
| -XX:-TraceClassUnloading | Trace unloading of classes. |
| -XX:-TraceLoaderConstraints | Trace recording of loader constraints. (Introduced in 6.) |
| -XX:+PerfDataSaveToFile | Saves jvmstat binary data on exit. |
| -XX:ParallelGCThreads=n | Sets the number of garbage collection threads in the young and old parallel garbage collectors. The default value varies with the platform on which the JVM is running. |
| -XX:+UseCompressedOops | Enables the use of compressed pointers (object references represented as 32 bit offsets instead of 64-bit pointers) for optimized 64-bit performance with Java heap sizes less than 32gb. |
| -XX:+AlwaysPreTouch | Pre-touch the Java heap during JVM initialization. Every page of the heap is thus demand-zeroed during initialization rather than incrementally during application execution. |
| -XX:AllocatePrefetchDistance=n | Sets the prefetch distance for object allocation. Memory about to be written with the value of new objects is prefetched into cache at this distance (in bytes) beyond the address of the last allocated object. Each Java thread has its own allocation point. The default value varies with the platform on which the JVM is running. |
| -XX:InlineSmallCode=n | Inline a previously compiled method only if its generated native code size is less than this. The default value varies with the platform on which the JVM is running. |
| -XX:MaxInlineSize=35 | Maximum bytecode size of a method to be inlined. |
| -XX:FreqInlineSize=n | Maximum bytecode size of a frequently executed method to be inlined. The default value varies with the platform on which the JVM is running. |
| -XX:LoopUnrollLimit=n | Unroll loop bodies with server compiler intermediate representation node count less than this value. The limit used by the server compiler is a function of this value, not the actual value. The default value varies with the platform on which the JVM is running. |
| -XX:InitialTenuringThreshold=7 | Sets the initial tenuring threshold for use in adaptive GC sizing in the parallel young collector. The tenuring threshold is the number of times an object survives a young collection before being promoted to the old, or tenured, generation. |
| -XX:MaxTenuringThreshold=n | Sets the maximum tenuring threshold for use in adaptive GC sizing. The current largest value is 15. The default value is 15 for the parallel collector and is 4 for CMS. |
| -Xloggc: |
Log GC verbose output to specified file. The verbose output is controlled by the normal verbose GC flags. |
| -XX:-UseGCLogFileRotation | Enabled GC log rotation, requires -Xloggc. |
| -XX:NumberOfGClogFiles=1 | Set the number of files to use when rotating logs, must be >= 1. The rotated log files will use the following naming scheme, |
| -XX:GCLogFileSize=8K | The size of the log file at which point the log will be rotated, must be >= 8K. |
1.3 JDK 1.8
https://docs.oracle.com/javase/8/docs/technotes/tools/unix/index.html
1.3.1 非标准选项（Hotspot VM）
| 选项 | 描述 |
|---|---|
| -Xmn | Sets the initial and maximum size (in bytes) of the heap for the young generation (nursery). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes.The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too small, then a lot of minor garbage collections will be performed. If the size is too large, then only full garbage collections will be performed, which can take a long time to complete. Oracle recommends that you keep the size for the young generation between a half and a quarter of the overall heap size. The following examples show how to set the initial and maximum size of young generation to 256 MB using various units: -Xmn256m -Xmn262144k -Xmn268435456 Instead of the -Xmn option to set both the initial and maximum size of the heap for the young generation, you can use -XX:NewSize to set the initial size and -XX:MaxNewSize to set the maximum size. |
| -Xms | Sets the minimum and the initial size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes.The following examples show how to set the size of allocated memory to 6 MB using various units: -Xms6291456 -Xms6144k -Xms6m If you do not set this option, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation. The initial size of the heap for the young generation can be set using the -Xmn option or the -XX:NewSize option.Note that the -XX:InitalHeapSize option can also be used to set the initial heap size. If it appears after -Xms on the command line, then the initial heap size gets set to the value specified with -XX:InitalHeapSize. |
| -Xmx | Specifies the maximum size (in bytes) of the memory allocation pool in bytes. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -Xms and -Xmx are often set to the same value. See the section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at [http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html](http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html).The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units: -Xmx83886080 -Xmx81920k -Xmx80m The -Xmx option is equivalent to -XX:MaxHeapSize. |
| -Xss | Sets the thread stack size (in bytes). Append the letter k or K to indicate KB, m or M to indicate MB, g or G to indicate GB. The default value depends on the platform:- Linux/ARM (32-bit): 320 KB - Linux/i386 (32-bit): 320 KB - Linux/x64 (64-bit): 1024 KB - macOS (64-bit): 1024 KB - Oracle Solaris/i386 (32-bit): 320 KB - Oracle Solaris/x64 (64-bit): 1024 KB - Windows: The default value depends on virtual memory The following examples set the thread stack size to 1024 KB in different units: -Xss1m -Xss1024k -Xss1048576 This option is similar to -XX:ThreadStackSize. |
1.3.2 Advanced Runtime Options
These options control the runtime behavior of the Java HotSpot VM.
| 选项 | 说明 |
|---|---|
| -XX:+CheckEndorsedAndExtDirs | Enables the option to prevent the java command from running a Java application if it uses the endorsed-standards override mechanism or the extension mechanism. This option checks if an application is using one of these mechanisms by checking the following:- The java.ext.dirs or java.endorsed.dirs system property is set.- The lib/endorsed directory exists and is not empty.- The lib/ext directory contains any JAR files other than those of the JDK.- The system-wide platform-specific extension directory contains any JAR files. |
| -XX:+DisableAttachMechanism | Enables the option that disables the mechanism that lets tools attach to the JVM. By default, this option is disabled, meaning that the attach mechanism is enabled and you can use tools such as jcmd, jstack, jmap, and jinfo. |
| -XX:ErrorFile=filename | Specifies the path and file name to which error data is written when an irrecoverable error occurs. By default, this file is created in the current working directory and named hs_err_pidpid.log where pid is the identifier of the process that caused the error. The following example shows how to set the default log file (note that the identifier of the process is specified as %p):-XX:ErrorFile=./hs_err_pid%p.log The following example shows how to set the error log to /var/log/java/java_error.log:-XX:ErrorFile=/var/log/java/java_error.log If the file cannot be created in the specified directory (due to insufficient space, permission problem, or another issue), then the file is created in the temporary directory for the operating system. The temporary directory is /tmp. |
| -XX:+FailOverToOldVerifier | Enables automatic failover to the old verifier when the new type checker fails. By default, this option is disabled and it is ignored (that is, treated as disabled) for classes with a recent bytecode version. You can enable it for classes with older versions of the bytecode. |
| -XX:+FlightRecorder | Enables the use of the Java Flight Recorder (JFR) during the runtime of the application. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option as follows:java -XX:+UnlockCommercialFeatures -XX:+FlightRecorder If this option is not provided, Java Flight Recorder can still be enabled in a running JVM by providing the appropriate jcmd diagnostic commands. |
| -XX:-FlightRecorder | Disables the use of the Java Flight Recorder (JFR) during the runtime of the application. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option as follows:java -XX:+UnlockCommercialFeatures -XX:-FlightRecorder If this option is provided, Java Flight Recorder cannot be enabled in a running JVM. |
| -XX:FlightRecorderOptions=parameter=value | Sets the parameters that control the behavior of JFR. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option. This option can be used only when JFR is enabled (that is, the -XX:+FlightRecorder option is specified).The following list contains all available JFR parameters: defaultrecording={true|false} Specifies whether a default continuous recording should be started for the Java application. By default, this parameter is set to false. To start a default recording automatically, set the parameter to true.disk={true|false} Specifies whether to write temporary data to the disk repository. By default, this parameter is set to false. To enable it, set the parameter to true.dumponexit={true|false} Specifies whether a dump file of JFR data should be generated when the JVM terminates in a controlled manner. By default, this parameter is set to false (dump file on exit is not generated). To enable it, set the parameter to true, and also set defaultrecording=true.The dump file is written to the location defined by the dumponexitpath parameter.dumponexitpath=path Specifies the path and name of the dump file with JFR data that is created when the JVM exits in a controlled manner if you set the dumponexit=true parameter. Setting the path makes sense only if you also set defaultrecording=true.If the specified path is a directory, the JVM assigns a file name that shows the creation date and time. If the specified path includes a file name and if that file already exists, the JVM creates a new file by appending the date and time stamp to the specified file name. globalbuffersize=size Specifies the total amount of primary memory (in bytes) used for data retention. Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. By default, the size is set to 462848 bytes.loglevel={quiet|error|warning|info|debug|trace} Specify the amount of data written to the log file by JFR. By default, it is set to info.maxage=time Specifies the maximum age of disk data to keep for the default recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 30s means 30 seconds). By default, the maximum age is set to 15 minutes (15m).This parameter is valid only if you set the disk=true parameter.maxchunksize=size Specifies the maximum size (in bytes) of the data chunks in a recording. Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. By default, the maximum size of data chunks is set to 12 MB.maxsize=size Specifies the maximum size (in bytes) of disk data to keep for the default recording. Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. By default, the maximum size of disk data is not limited, and this parameter is set to 0.This parameter is valid only if you set the disk=true parameter.repository=path Specifies the repository (a directory) for temporary disk storage. By default, the system's temporary directory is used. samplethreads={true|false} Specifies whether thread sampling is enabled. Thread sampling occurs only if the sampling event is enabled along with this parameter. By default, this parameter is enabled. settings=path Specifies the path and name of the event settings file (of type JFC). By default, the default.jfc file is used, which is located in JAVA_HOME/jre/lib/jfr.stackdepth=depth Stack depth for stack traces by JFR. By default, the depth is set to 64 method calls. The maximum is 2048, minimum is 1. threadbuffersize=size Specifies the per-thread local buffer size (in bytes). Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. Higher values for this parameter allow more data gathering without contention to flush it to the global storage. It can increase application footprint in a thread-rich environment. By default, the local buffer size is set to 5 KB.You can specify values for multiple parameters by separating them with a comma. For example, to instruct JFR to write a continuous recording to disk, and set the maximum size of data chunks to 10 MB, specify the following: -XX:FlightRecorderOptions=defaultrecording=true,disk=true,maxchunksize=10M |
| -XX:LargePageSizeInBytes=size | On Solaris, sets the maximum size (in bytes) for large pages used for Java heap. The size argument must be a power of 2 (2, 4, 8, 16, ...). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for large pages automatically.The following example illustrates how to set the large page size to 4 megabytes (MB): -XX:LargePageSizeInBytes=4m |
| -XX:MaxDirectMemorySize=size |
Sets the maximum total size (in bytes) of the New I/O (the java.nio package) direct-buffer allocations. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for NIO direct-buffer allocations automatically.The following examples illustrate how to set the NIO size to 1024 KB in different units: -XX:MaxDirectMemorySize=1m -XX:MaxDirectMemorySize=1024k -XX:MaxDirectMemorySize=1048576 |
| -XX:NativeMemoryTracking=mode | Specifies the mode for tracking JVM native memory usage. Possible mode arguments for this option include the following: off - Do not track JVM native memory usage. This is the default behavior if you do not specify the -XX:NativeMemoryTracking option.summary - Only track memory usage by JVM subsystems, such as Java heap, class, code, and thread. detail - In addition to tracking memory usage by JVM subsystems, track memory usage by individual CallSite, individual virtual memory region and its committed regions. |
| -XX:ObjectAlignmentInBytes=alignment | Sets the memory alignment of Java objects (in bytes). By default, the value is set to 8 bytes. The specified value should be a power of two, and must be within the range of 8 and 256 (inclusive). This option makes it possible to use compressed pointers with large Java heap sizes. The heap size limit in bytes is calculated as: 4GB * ObjectAlignmentInBytesNote: As the alignment value increases, the unused space between objects will also increase. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes. |
| -XX:OnError=string | Sets a custom command or a series of semicolon-separated commands to run when an irrecoverable error occurs. If the string contains spaces, then it must be enclosed in quotation marks. The following example shows how the -XX:OnError option can be used to run the gcore command to create the core image, and the debugger is started to attach to the process in case of an irrecoverable error (the %p designates the current process):-XX:OnError="gcore %p;dbx - %p" |
| -XX:OnOutOfMemoryError=string | Sets a custom command or a series of semicolon-separated commands to run when an OutOfMemoryError exception is first thrown. If the string contains spaces, then it must be enclosed in quotation marks. For an example of a command string, see the description of the -XX:OnError option. |
| -XX:+PerfDataSaveToFile | If enabled, saves jstat(1) binary data when the Java application exits. This binary data is saved in a file named hsperfdata_``<pid>, where <pid> is the process identifier of the Java application you ran. Use jstat to display the performance data contained in this file as follows:<br>jstat -class file:///<path>/hsperfdata_<pid><br>jstat -gc file:///<path>/hsperfdata_<pid><br> |
| -XX:-PreferContainerQuotaForCPUCount | If true, then calculate the container CPU availability based on the value of its CPU CFS (Completely Fair Scheduler) quota (if set). If false, then use the CPU shares value instead, provided it is less than the CPU quota value. The VM uses the container CPU availability value to calculate the number of available processors if you haven't specified it with the -XX:ActiveProcessorCount option.Note: The CPU count will never exceed the number of active processors available to the process. Active processors may have been limited by having them disabled or by selecting a cpuset (a list of CPUs; see the PrintContainerInfo option) for a container. |
| -XX:+PrintCommandLineFlags | Enables printing of ergonomically selected JVM flags that appeared on the command line. It can be useful to know the ergonomic values set by the JVM, such as the heap space size and the selected garbage collector. By default, this option is disabled and flags are not printed. |
| -XX:+PrintContainerInfo | Prints the following information about the container: - cpuset.cpus: A comma-separated list or hyphen-separated range of CPUs or cores a container can use provided that you have more than one CPU or core. For example, the value 0-3 means that the container can use the first, second, third, and fourth CPU.- cpuset.mems: A comma-separated list or hyphen-separated range of memory nodes (MEMs) in which to allow execution; running of applications; only effective on non-uniform memory access (NUMA) systems.- CPU Shares: The amount of CPU shares available to the process.- CPU Quota: The number of milliseconds per period process is guaranteed to run.- CPU Period: The CPU Completely Fair Scheduler (CFS) period, in ms.- OSContainer::active_processor_count: Number of active processors for the VM to use.- Memory Limit: The maximum amount of memory the container can use, in bytes.- Memory Soft Limit: The maximum amount of memory a container should use, in bytes; this limit is smaller than Memory Limit, but because this is a soft limit, the container may exceed it.- Memory Usage: The amount of used memory for this process, in bytes.- Maximum Memory Usage: The maximum amount of used memory for this process, in bytes. |
| -XX:+PrintNMTStatistics | Enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled (see -XX:NativeMemoryTracking). By default, this option is disabled and native memory tracking data is not printed. |
| -XX:+RelaxAccessControlCheck | Decreases the amount of access control checks in the verifier. By default, this option is disabled, and it is ignored (that is, treated as disabled) for classes with a recent bytecode version. You can enable it for classes with older versions of the bytecode. |
| -XX:+ResourceManagement | Enables the use of Resource Management during the runtime of the application. This is a commercial feature that requires you to also specify the -XX:+UnlockCommercialFeatures option as follows:java -XX:+UnlockCommercialFeatures -XX:+ResourceManagement |
| -XX:ResourceManagementSampleInterval=value (milliseconds) | Sets the parameter that controls the sampling interval for Resource Management measurements, in milliseconds. This option can be used only when Resource Management is enabled (that is, the -XX:+ResourceManagement option is specified). |
| -XX:SharedArchiveFile=path | Specifies the path and name of the class data sharing (CDS) archive file |
| -XX:SharedClassListFile=file_name | Specifies the text file that contains the names of the class files to store in the class data sharing (CDS) archive. This file contains the full name of one class file per line, except slashes (/) replace dots (.). For example, to specify the classes java.lang.Object and hello.Main, create a text file that contains the following two lines:<br><br>java/lang/Object<br>hello/Main<br>The class files that you specify in this text file should include the classes that are commonly used by the application. They may include any classes from the application, extension, or bootstrap class paths. |
| -XX:+ShowMessageBoxOnError | Enables displaying of a dialog box when the JVM experiences an irrecoverable error. This prevents the JVM from exiting and keeps the process active so that you can attach a debugger to it to investigate the cause of the error. By default, this option is disabled. |
| -XX:StartFlightRecording=parameter=value | Starts a JFR recording for the Java application. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option. This option is equivalent to the JFR.start diagnostic command that starts a recording during runtime. You can set the following parameters when starting a JFR recording:compress={true|false} Specifies whether to compress the JFR recording log file (of type JFR) on the disk using the gzip file compression utility. This parameter is valid only if the filename parameter is specified. By default it is set to false (recording is not compressed). To enable compression, set the parameter to true.defaultrecording={true|false} Specifies whether the recording is a continuous background recording or if it runs for a limited time. By default, this parameter is set to false (recording runs for a limited time). To make the recording run continuously, set the parameter to true.delay=time Specifies the delay between the Java application launch time and the start of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 10m means 10 minutes). By default, there is no delay, and this parameter is set to 0.dumponexit={true|false} Specifies whether a dump file of JFR data should be generated when the JVM terminates in a controlled manner. By default, this parameter is set to false (dump file on exit is not generated). To enable it, set the parameter to true.The dump file is written to the location defined by the filename parameter.Example: -XX:StartFlightRecording=name=test,filename=D:\test.jfr,dumponexit=true duration=time Specifies the duration of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 5h means 5 hours). By default, the duration is not limited, and this parameter is set to 0.filename=path Specifies the path and name of the JFR recording log file. name=identifier Specifies the identifier for the JFR recording. By default, it is set to Recording x.maxage=time Specifies the maximum age of disk data to keep for the default recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 30s means 30 seconds). By default, the maximum age is set to 15 minutes (15m).maxsize=size Specifies the maximum size (in bytes) of disk data to keep for the default recording. Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. By default, the maximum size of disk data is not limited, and this parameter is set to 0.settings=path Specifies the path and name of the event settings file (of type JFC). By default, the default.jfc file is used, which is located in JAVA_HOME/jre/lib/jfr.You can specify values for multiple parameters by separating them with a comma. For example, to save the recording to test.jfr in the current working directory, and instruct JFR to compress the log file, specify the following: -XX:StartFlightRecording=filename=test.jfr,compress=true |
| -XX:ThreadStackSize=size | Sets the Java thread stack size (in kilobytes). Use of a scaling suffix, such as k, results in the scaling of the kilobytes value so that -XX:ThreadStackSize=1k sets the Java thread stack size to 1024*1024 bytes or 1 megabyte. The default value depends on the platform:- Linux/ARM (32-bit): 320 KB - Linux/i386 (32-bit): 320 KB - Linux/x64 (64-bit): 1024 KB - macOS (64-bit): 1024 KB - Oracle Solaris/i386 (32-bit): 320 KB - Oracle Solaris/x64 (64-bit): 1024 KB The following examples show how to set the thread stack size to 1 megabyte in different units: -XX:ThreadStackSize=1k -XX:ThreadStackSize=1024 This option is similar to -Xss. |
| -XX:+TraceClassLoading | Enables tracing of classes as they are loaded. By default, this option is disabled and classes are not traced. |
| -XX:+TraceClassLoadingPreorder | Enables tracing of all loaded classes in the order in which they are referenced. By default, this option is disabled and classes are not traced. |
| -XX:+TraceClassResolution | Enables tracing of constant pool resolutions. By default, this option is disabled and constant pool resolutions are not traced. |
| -XX:+TraceClassUnloading | Enables tracing of classes as they are unloaded. By default, this option is disabled and classes are not traced. |
| -XX:+TraceLoaderConstraints | Enables tracing of the loader constraints recording. By default, this option is disabled and loader constraints recording is not traced. |
| -XX:+UnlockCommercialFeatures | Enables the use of commercial features. Commercial features are included with Oracle Java SE Advanced or Oracle Java SE Suite packages, as defined on the Java SE Products page at [http://www.oracle.com/technetwork/java/javase/terms/products/index.html](http://www.oracle.com/technetwork/java/javase/terms/products/index.html)By default, this option is disabled and the JVM runs without the commercial features. Once they were enabled for a JVM process, it is not possible to disable their use for that process. If this option is not provided, commercial features can still be unlocked in a running JVM by using the appropriate jcmd diagnostic commands. |
| -XX:+UseAltSigs | Enables the use of alternative signals instead of SIGUSR1 and SIGUSR2 for JVM internal signals. By default, this option is disabled and alternative signals are not used. This option is equivalent to -Xusealtsigs. |
| -XX:+UseAppCDS | Enables application class data sharing (AppCDS). To use AppCDS, you must also specify values for the options -XX:SharedClassListFile and -XX:SharedArchiveFile during both CDS dump time (see the option -Xshare:dump) and application run time.This is a commercial feature that requires you to also specify the -XX:+UnlockCommercialFeatures option. This is also an experimental feature; it may change in future releases.See "Application Class Data Sharing". |
| -XX:-UseBiasedLocking | Disables the use of biased locking. Some applications with significant amounts of uncontended synchronization may attain significant speedups with this flag enabled, whereas applications with certain patterns of locking may see slowdowns. For more information about the biased locking technique, see the example in Java Tuning White Paper at [http://www.oracle.com/technetwork/java/tuning-139912.html#section4.2.5](http://www.oracle.com/technetwork/java/tuning-139912.html#section4.2.5)By default, this option is enabled. |
| -XX:-UseCompressedOops | Disables the use of compressed pointers. By default, this option is enabled, and compressed pointers are used when Java heap sizes are less than 32 GB. When this option is enabled, object references are represented as 32-bit offsets instead of 64-bit pointers, which typically increases performance when running the application with Java heap sizes less than 32 GB. This option works only for 64-bit JVMs. It is also possible to use compressed pointers when Java heap sizes are greater than 32GB. See the -XX:ObjectAlignmentInBytes option. |
| -XX:-UseContainerSupport | The VM provides automatic container detection support, which enables the VM to determine the amount of memory and number of processors that are available to a Java process running in docker containers. It uses this information to allocate system resources. This support is only available on Linux x64 platforms. If supported, then the default value for this flag is true and container support is enabled by default. You can disable it with -XX:-UseContainerSupport. |
| -XX:+UseHugeTLBFS | This option for Linux is the equivalent of specifying -XX:+UseLargePages. This option is disabled by default. This option pre-allocates all large pages up-front, when memory is reserved; consequently the JVM cannot dynamically grow or shrink large pages memory areas; see -XX:UseTransparentHugePages if you want this behavior.For more information, see "Large Pages". |
| -XX:+UseLargePages | Enables the use of large page memory. By default, this option is disabled and large page memory is not used. For more information, see "Large Pages". |
| -XX:+UseMembar | Enables issuing of membars on thread state transitions. This option is disabled by default on all platforms except ARM servers, where it is enabled. (It is recommended that you do not disable this option on ARM servers.) |
| -XX:+UsePerfData | Enables the perfdata feature. This option is enabled by default to allow JVM monitoring and performance testing. Disabling it suppresses the creation of the hsperfdata_userid directories. To disable the perfdata feature, specify -XX:-UsePerfData. |
| -XX:+UseTransparentHugePages | On Linux, enables the use of large pages that can dynamically grow or shrink. This option is disabled by default. You may encounter performance problems with transparent huge pages as the OS moves other pages around to create huge pages; this option is made available for experimentation. For more information, see "Large Pages". |
-XX:+AllowUserSignalHandlers |
Enables installation of signal handlers by the application. By default, this option is disabled and the application is not allowed to install signal handlers. |
1.3.3 Advanced JIT Compiler Options
These options control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM.
| 选项 | 说明 |
|---|---|
| -XX:+AggressiveOpts | Enables the use of aggressive performance optimization features, which are expected to become default in upcoming releases. By default, this option is disabled and experimental performance features are not used. |
| -XX:AllocateInstancePrefetchLines=lines | Sets the number of lines to prefetch ahead of the instance allocation pointer. By default, the number of lines to prefetch is set to 1: -XX:AllocateInstancePrefetchLines=1 Only the Java HotSpot Server VM supports this option. |
| -XX:AllocatePrefetchDistance=size | Sets the size (in bytes) of the prefetch distance for object allocation. Memory about to be written with the value of new objects is prefetched up to this distance starting from the address of the last allocated object. Each Java thread has its own allocation point. Negative values denote that prefetch distance is chosen based on the platform. Positive values are bytes to prefetch. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is set to -1.The following example shows how to set the prefetch distance to 1024 bytes: -XX:AllocatePrefetchDistance=1024 Only the Java HotSpot Server VM supports this option. |
| -XX:AllocatePrefetchInstr=instruction | Sets the prefetch instruction to prefetch ahead of the allocation pointer. Only the Java HotSpot Server VM supports this option. Possible values are from 0 to 3. The actual instructions behind the values depend on the platform. By default, the prefetch instruction is set to 0: -XX:AllocatePrefetchInstr=0 Only the Java HotSpot Server VM supports this option. |
| -XX:AllocatePrefetchLines=lines | Sets the number of cache lines to load after the last object allocation by using the prefetch instructions generated in compiled code. The default value is 1 if the last allocated object was an instance, and 3 if it was an array. The following example shows how to set the number of loaded cache lines to 5: -XX:AllocatePrefetchLines=5 Only the Java HotSpot Server VM supports this option. |
| -XX:AllocatePrefetchStepSize=size | Sets the step size (in bytes) for sequential prefetch instructions. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the step size is set to 16 bytes:-XX:AllocatePrefetchStepSize=16 Only the Java HotSpot Server VM supports this option. |
| -XX:AllocatePrefetchStyle=style | Sets the generated code style for prefetch instructions. The style argument is an integer from 0 to 3: 0 - Do not generate prefetch instructions. 1 - Execute prefetch instructions after each allocation. This is the default parameter. 2 - Use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed. 3 - Use BIS instruction on SPARC for allocation prefetch. Only the Java HotSpot Server VM supports this option. |
| -XX:+BackgroundCompilation | Enables background compilation. This option is enabled by default. To disable background compilation, specify -XX:-BackgroundCompilation (this is equivalent to specifying -Xbatch). |
| -XX:CICompilerCount=threads | Sets the number of compiler threads to use for compilation. By default, the number of threads is set to 2 for the server JVM, to 1 for the client JVM, and it scales to the number of cores if tiered compilation is used. The following example shows how to set the number of threads to 2: -XX:CICompilerCount=2 |
| -XX:CodeCacheMinimumFreeSpace=size | Sets the minimum free space (in bytes) required for compilation. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. When less than the minimum free space remains, compiling stops. By default, this option is set to 500 KB. The following example shows how to set the minimum free space to 1024 MB:-XX:CodeCacheMinimumFreeSpace=1024m |
| -XX:CompileCommand=command,method[,option] | Specifies a command to perform on a method. For example, to exclude the indexOf() method of the String class from being compiled, use the following:-XX:CompileCommand=exclude,java/lang/String.indexOf Note that the full class name is specified, including all packages and subpackages separated by a slash ( /). For easier cut and paste operations, it is also possible to use the method name format produced by the -XX:+PrintCompilation and -XX:+LogCompilation options:-XX:CompileCommand=exclude,java.lang.String::indexOf If the method is specified without the signature, the command will be applied to all methods with the specified name. However, you can also specify the signature of the method in the class file format. In this case, you should enclose the arguments in quotation marks, because otherwise the shell treats the semicolon as command end. For example, if you want to exclude only the indexOf(String) method of the String class from being compiled, use the following:-XX:CompileCommand="exclude,java/lang/String.indexOf,(Ljava/lang/String;)I" You can also use the asterisk () as a wildcard for class and method names. For example, to exclude all indexOf() methods in all classes from being compiled, use the following:-XX:CompileCommand=exclude,.indexOf The commas and periods are aliases for spaces, making it easier to pass compiler commands through a shell. You can pass arguments to -XX:CompileCommand using spaces as separators by enclosing the argument in quotation marks:-XX:CompileCommand="exclude java/lang/String indexOf" Note that after parsing the commands passed on the command line using the -XX:CompileCommand options, the JIT compiler then reads commands from the .hotspot_compiler file. You can add commands to this file or specify a different file using the -XX:CompileCommandFile option.To add several commands, either specify the -XX:CompileCommand option multiple times, or separate each argument with the newline separator (\n). The following commands are available:break Set a breakpoint when debugging the JVM to stop at the beginning of compilation of the specified method. compileonly Exclude all methods from compilation except for the specified method. As an alternative, you can use the -XX:CompileOnly option, which allows to specify several methods.dontinline Prevent inlining of the specified method. exclude Exclude the specified method from compilation. help Print a help message for the -XX:CompileCommand option.inline Attempt to inline the specified method. log Exclude compilation logging (with the -XX:+LogCompilation option) for all methods except for the specified method. By default, logging is performed for all compiled methods.option This command can be used to pass a JIT compilation option to the specified method in place of the last argument (option). The compilation option is set at the end, after the method name. For example, to enable the BlockLayoutByFrequency option for the append() method of the StringBuffer class, use the following:-XX:CompileCommand=option,java/lang/StringBuffer.append,BlockLayoutByFrequency You can specify multiple compilation options, separated by commas or spaces. Print generated assembler code after compilation of the specified method. quiet Do not print the compile commands. By default, the commands that you specify with the - XX:CompileCommand option are printed; for example, if you exclude from compilation the indexOf() method of the String class, then the following will be printed to standard output:CompilerOracle: exclude java/lang/String.indexOf You can suppress this by specifying the -XX:CompileCommand=quiet option before other -XX:CompileCommand options. |
| -XX:CompileCommandFile=filename | Sets the file from which JIT compiler commands are read. By default, the .hotspot_compiler file is used to store commands performed by the JIT compiler.Each line in the command file represents a command, a class name, and a method name for which the command is used. For example, this line prints assembly code for the toString() method of the String class:print java/lang/String toString For more information about specifying the commands for the JIT compiler to perform on methods, see the -XX:CompileCommand option. |
| -XX:CompileOnly=methods | Sets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods will be compiled. Specify each method with the full class name (including the packages and subpackages). For example, to compile only the length() method of the String class and the size() method of the List class, use the following:-XX:CompileOnly=java/lang/String.length,java/util/List.size Note that the full class name is specified, including all packages and subpackages separated by a slash ( /). For easier cut and paste operations, it is also possible to use the method name format produced by the -XX:+PrintCompilation and -XX:+LogCompilation options:-XX:CompileOnly=java.lang.String::length,java.util.List::size Although wildcards are not supported, you can specify only the class or package name to compile all methods in that class or package, as well as specify just the method to compile methods with this name in any class: -XX:CompileOnly=java/lang/String -XX:CompileOnly=java/lang -XX:CompileOnly=.length |
| -XX:CompileThreshold=invocations | Sets the number of interpreted method invocations before compilation. By default, in the server JVM, the JIT compiler performs 10,000 interpreted method invocations to gather information for efficient compilation. For the client JVM, the default setting is 1,500 invocations. This option is ignored when tiered compilation is enabled; see the option -XX:+TieredCompilation. The following example shows how to set the number of interpreted method invocations to 5,000:-XX:CompileThreshold=5000 You can completely disable interpretation of Java methods before compilation by specifying the -Xcomp option. |
| -XX:+DoEscapeAnalysis | Enables the use of escape analysis. This option is enabled by default. To disable the use of escape analysis, specify -XX:-DoEscapeAnalysis. Only the Java HotSpot Server VM supports this option. |
| -XX:InitialCodeCacheSize=size | Sets the initial code cache size (in bytes). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is set to 500 KB. The initial code cache size should be not less than the system's minimal memory page size. The following example shows how to set the initial code cache size to 32 KB:-XX:InitialCodeCacheSize=32k |
| -XX:+Inline | Enables method inlining. This option is enabled by default to increase performance. To disable method inlining, specify -XX:-Inline. |
| -XX:InlineSmallCode=size | Sets the maximum code size (in bytes) for compiled methods that should be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. Only compiled methods with the size smaller than the specified size will be inlined. By default, the maximum code size is set to 1000 bytes:-XX:InlineSmallCode=1000 |
| -XX:+LogCompilation | Enables logging of compilation activity to a file named hotspot.log in the current working directory. You can specify a different log file path and name using the -XX:LogFile option.By default, this option is disabled and compilation activity is not logged. The -XX:+LogCompilation option has to be used together with the -XX:+UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.You can enable verbose diagnostic output with a message printed to the console every time a method is compiled by using the -XX:+PrintCompilation option. |
| -XX:MaxInlineSize=size | Sets the maximum bytecode size (in bytes) of a method to be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the maximum bytecode size is set to 35 bytes:-XX:MaxInlineSize=35 |
| -XX:MaxNodeLimit=nodes | Sets the maximum number of nodes to be used during single method compilation. By default, the maximum number of nodes is set to 65,000: -XX:MaxNodeLimit=65000 |
| -XX:MaxTrivialSize=size | Sets the maximum bytecode size (in bytes) of a trivial method to be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the maximum bytecode size of a trivial method is set to 6 bytes:-XX:MaxTrivialSize=6 |
| -XX:+OptimizeStringConcat | Enables the optimization of String concatenation operations. This option is enabled by default. To disable the optimization of String concatenation operations, specify -XX:-OptimizeStringConcat. Only the Java HotSpot Server VM supports this option. |
| -XX:+PrintAssembly | Enables printing of assembly code for bytecoded and native methods by using the external disassembler.so library. This enables you to see the generated code, which may help you to diagnose performance issues.By default, this option is disabled and assembly code is not printed. The -XX:+PrintAssembly option has to be used together with the -XX:+UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options. |
| -XX:+PrintCompilation | Enables verbose diagnostic output from the JVM by printing a message to the console every time a method is compiled. This enables you to see which methods actually get compiled. By default, this option is disabled and diagnostic output is not printed. You can also log compilation activity to a file by using the -XX:+LogCompilation option. |
| -XX:+PrintInlining | Enables printing of inlining decisions. This enables you to see which methods are getting inlined. By default, this option is disabled and inlining information is not printed. The -XX:+PrintInlining option has to be used together with the -XX:+UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options. |
| -XX:ReservedCodeCacheSize=size | Sets the maximum code cache size (in bytes) for JIT-compiled code. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default maximum code cache size is 240 MB; if you disable tiered compilation with the option -XX:-TieredCompilation, then the default size is 48 MB. This option has a limit of 2 GB; otherwise, an error is generated. The maximum code cache size should not be less than the initial code cache size; see the option -XX:InitialCodeCacheSize. This option is equivalent to -Xmaxjitcodesize. |
-XX:RTMAbortRatio=abort_ratio |
The RTM abort ratio is specified as a percentage (%) of all executed RTM transactions. If a number of aborted transactions becomes greater than this ratio, then the compiled code will be deoptimized. This ratio is used when the -XX:+UseRTMDeopt option is enabled. The default value of this option is 50. This means that the compiled code will be deoptimized if 50% of all transactions are aborted. |
| -XX:RTMRetryCount=number_of_retries | RTM locking code will be retried, when it is aborted or busy, the number of times specified by this option before falling back to the normal locking mechanism. The default value for this option is 5. The -XX:UseRTMLocking option must be enabled. |
| -XX:-TieredCompilation | Disables the use of tiered compilation. By default, this option is enabled. Only the Java HotSpot Server VM supports this option. |
| -XX:+UseAES | Enables hardware-based AES intrinsics for Intel, AMD, and SPARC hardware. Intel Westmere (2010 and newer), AMD Bulldozer (2011 and newer), and SPARC (T4 and newer) are the supported hardware. UseAES is used in conjunction with UseAESIntrinsics. |
| -XX:+UseAESIntrinsics | UseAES and UseAESIntrinsics flags are enabled by default and are supported only for Java HotSpot Server VM 32-bit and 64-bit. To disable hardware-based AES intrinsics, specify -XX:-UseAES -XX:-UseAESIntrinsics. For example, to enable hardware AES, use the following flags:-XX:+UseAES -XX:+UseAESIntrinsics To support UseAES and UseAESIntrinsics flags for 32-bit and 64-bit use -server option to choose Java HotSpot Server VM. These flags are not supported on Client VM. |
| -XX:+UseCodeCacheFlushing | Enables flushing of the code cache before shutting down the compiler. This option is enabled by default. To disable flushing of the code cache before shutting down the compiler, specify -XX:-UseCodeCacheFlushing. |
| -XX:+UseCondCardMark | Enables checking of whether the card is already marked before updating the card table. This option is disabled by default and should only be used on machines with multiple sockets, where it will increase performance of Java applications that rely heavily on concurrent operations. Only the Java HotSpot Server VM supports this option. |
| -XX:+UseRTMDeopt | Auto-tunes RTM locking depending on the abort ratio. This ratio is specified by -XX:RTMAbortRatio option. If the number of aborted transactions exceeds the abort ratio, then the method containing the lock will be deoptimized and recompiled with all locks as normal locks. This option is disabled by default. The -XX:+UseRTMLocking option must be enabled. |
| -XX:+UseRTMLocking | Generate Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism as the fallback handler. This option is disabled by default. Options related to RTM are only available for the Java HotSpot Server VM on x86 CPUs that support Transactional Synchronization Extensions (TSX). RTM is part of Intel's TSX, which is an x86 instruction set extension and facilitates the creation of multithreaded applications. RTM introduces the new instructions XBEGIN, XABORT, XEND, and XTEST. The XBEGIN and XEND instructions enclose a set of instructions to run as a transaction. If no conflict is found when running the transaction, the memory and register modifications are committed together at the XEND instruction. The XABORT instruction can be used to explicitly abort a transaction and the XEND instruction to check if a set of instructions are being run in a transaction.A lock on a transaction is inflated when another thread tries to access the same transaction, thereby blocking the thread that did not originally request access to the transaction. RTM requires that a fallback set of operations be specified in case a transaction aborts or fails. An RTM lock is a lock that has been delegated to the TSX's system. RTM improves performance for highly contended locks with low conflict in a critical region (which is code that must not be accessed by more than one thread concurrently). RTM also improves the performance of coarse-grain locking, which typically does not perform well in multithreaded applications. (Coarse-grain locking is the strategy of holding locks for long periods to minimize the overhead of taking and releasing locks, while fine-grained locking is the strategy of trying to achieve maximum parallelism by locking only when necessary and unlocking as soon as possible.) Also, for lightly contended locks that are used by different threads, RTM can reduce false cache line sharing, also known as cache line ping-pong. This occurs when multiple threads from different processors are accessing different resources, but the resources share the same cache line. As a result, the processors repeatedly invalidate the cache lines of other processors, which forces them to read from main memory instead of their cache. |
| -XX:+UseSHA | Enables hardware-based intrinsics for SHA crypto hash functions for SPARC hardware. UseSHA is used in conjunction with the UseSHA1Intrinsics, UseSHA256Intrinsics, and UseSHA512Intrinsics options.The UseSHA and UseSHA*Intrinsics flags are enabled by default, and are supported only for Java HotSpot Server VM 64-bit on SPARC T4 and newer.This feature is only applicable when using the sun.security.provider.Sun provider for SHA operations.To disable all hardware-based SHA intrinsics, specify -XX:-UseSHA. To disable only a particular SHA intrinsic, use the appropriate corresponding option. For example: -XX:-UseSHA256Intrinsics. |
| -XX:+UseSHA1Intrinsics | Enables intrinsics for SHA-1 crypto hash function. |
| -XX:+UseSHA256Intrinsics | Enables intrinsics for SHA-224 and SHA-256 crypto hash functions. |
| -XX:+UseSHA512Intrinsics | Enables intrinsics for SHA-384 and SHA-512 crypto hash functions. |
| -XX:+UseSuperWord | Enables the transformation of scalar operations into superword operations. This option is enabled by default. To disable the transformation of scalar operations into superword operations, specify -XX:-UseSuperWord. Only the Java HotSpot Server VM supports this option. |
1.3.4 Advanced Serviceability Options
These options provide the ability to gather system information and perform extensive debugging.
| 选项 | 说明 |
|---|---|
| -XX:+ExtendedDTraceProbes | Enables additional dtrace tool probes that impact the performance. By default, this option is disabled and dtrace performs only standard probes. |
| -XX:+HeapDumpOnOutOfMemoryError | Enables the dumping of the Java heap to a file in the current directory by using the heap profiler (HPROF) when a java.lang.OutOfMemoryError exception is thrown. You can explicitly set the heap dump file path and name using the -XX:HeapDumpPath option. By default, this option is disabled and the heap is not dumped when an OutOfMemoryError exception is thrown. |
| -XX:HeapDumpPath=path | Sets the path and file name for writing the heap dump provided by the heap profiler (HPROF) when the -XX:+HeapDumpOnOutOfMemoryError option is set. By default, the file is created in the current working directory, and it is named java_pidpid.hprof where pid is the identifier of the process that caused the error. The following example shows how to set the default file explicitly (%p represents the current process identificator):-XX:HeapDumpPath=./java_pid%p.hprof The following example shows how to set the heap dump file to /var/log/java/java_heapdump.hprof:-XX:HeapDumpPath=/var/log/java/java_heapdump.hprof |
| -XX:LogFile=path | Sets the path and file name where log data is written. By default, the file is created in the current working directory, and it is named hotspot.log.The following example shows how to set the log file to /var/log/java/hotspot.log:-XX:LogFile=/var/log/java/hotspot.log |
| -XX:+PrintClassHistogram | Enables printing of a class instance histogram after a Control+C event (SIGTERM). By default, this option is disabled.Setting this option is equivalent to running the jmap -histo command, or the jcmd pid GC.class_histogram command, where pid is the current Java process identifier. |
| -XX:+PrintConcurrentLocks | Enables printing of java.util.concurrent locks after a Control+C event (SIGTERM). By default, this option is disabled.Setting this option is equivalent to running the jstack -l command or the jcmd pid Thread.print -l command, where pid is the current Java process identifier. |
| -XX:+UnlockDiagnosticVMOptions | Unlocks the options intended for diagnosing the JVM. By default, this option is disabled and diagnostic options are not available. |
1.3.5 Advanced Garbage Collection Options
These options control how garbage collection (GC) is performed by the Java HotSpot VM.
| 选项 | 说明 |
|---|---|
| -XX:ActiveProcessorCount=x | Overrides the number of CPUs that the VM uses to calculate the size of thread pools it uses for various operations such as Garbage Collection and ForkJoinPool. The VM normally determines the number of available processors from the operating system. This flag can be useful for partitioning CPU resources when running multiple Java processes in docker containers. This flag is honored even if UseContainerSupport is not enabled. See -XX:-UseContainerSupport for a description of enabling and disabling container support. |
| -XX:+AggressiveHeap | Enables Java heap optimization. This sets various parameters to be optimal for long-running jobs with intensive memory allocation, based on the configuration of the computer (RAM and CPU). By default, the option is disabled and the heap is not optimized. |
| -XX:+AlwaysPreTouch | Enables touching of every page on the Java heap during JVM initialization. This gets all pages into the memory before entering the main() method. The option can be used in testing to simulate a long-running system with all virtual memory mapped to physical memory. By default, this option is disabled and all pages are committed as JVM heap space fills. |
| -XX:+CMSClassUnloadingEnabled | Enables class unloading when using the concurrent mark-sweep (CMS) garbage collector. This option is enabled by default. To disable class unloading for the CMS garbage collector, specify -XX:-CMSClassUnloadingEnabled. |
| -XX:CMSExpAvgFactor=percent | Sets the percentage of time (0 to 100) used to weight the current sample when computing exponential averages for the concurrent collection statistics. By default, the exponential averages factor is set to 25%. The following example shows how to set the factor to 15%:-XX:CMSExpAvgFactor=15 |
| -XX:CMSInitiatingOccupancyFraction=percent | Sets the percentage of the old generation occupancy (0 to 100) at which to start a CMS collection cycle. The default value is set to -1. Any negative value (including the default) implies that -XX:CMSTriggerRatio is used to define the value of the initiating occupancy fraction.The following example shows how to set the occupancy fraction to 20%: -XX:CMSInitiatingOccupancyFraction=20 |
| -XX:+CMSScavengeBeforeRemark | Enables scavenging attempts before the CMS remark step. By default, this option is disabled. |
| -XX:CMSTriggerRatio=percent | Sets the percentage (0 to 100) of the value specified by -XX:MinHeapFreeRatio that is allocated before a CMS collection cycle commences. The default value is set to 80%.The following example shows how to set the occupancy fraction to 75%: -XX:CMSTriggerRatio=75 |
| -XX:ConcGCThreads=threads | Sets the number of threads used for concurrent GC. The default value depends on the number of CPUs available to the JVM. For example, to set the number of threads for concurrent GC to 2, specify the following option: -XX:ConcGCThreads=2 |
| -XX:+DisableExplicitGC | Enables the option that disables processing of calls to System.gc(). This option is disabled by default, meaning that calls to System.gc() are processed. If processing of calls to System.gc() is disabled, the JVM still performs GC when necessary. |
| -XX:+ExplicitGCInvokesConcurrent | Enables invoking of concurrent GC by using the System.gc() request. This option is disabled by default and can be enabled only together with the -XX:+UseConcMarkSweepGC option. |
| -XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses | Enables invoking of concurrent GC by using the System.gc() request and unloading of classes during the concurrent GC cycle. This option is disabled by default and can be enabled only together with the -XX:+UseConcMarkSweepGC option. |
| -XX:G1HeapRegionSize=size | Sets the size of the regions into which the Java heap is subdivided when using the garbage-first (G1) collector. The value can be between 1 MB and 32 MB. The default region size is determined ergonomically based on the heap size. The following example shows how to set the size of the subdivisions to 16 MB: -XX:G1HeapRegionSize=16m |
| -XX:+G1PrintHeapRegions | Enables the printing of information about which regions are allocated and which are reclaimed by the G1 collector. By default, this option is disabled. |
| -XX:G1ReservePercent=percent | Sets the percentage of the heap (0 to 50) that is reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector. By default, this option is set to 10%. The following example shows how to set the reserved heap to 20%: -XX:G1ReservePercent=20 |
| -XX:InitialHeapSize=size | Sets the initial size (in bytes) of the memory allocation pool. This value must be either 0, or a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. See the section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at [http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html](http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html).The following examples show how to set the size of allocated memory to 6 MB using various units: -XX:InitialHeapSize=6291456 -XX:InitialHeapSize=6144k -XX:InitialHeapSize=6m If you set this option to 0, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation. The size of the heap for the young generation can be set using the -XX:NewSize option.Note that the -Xms option sets both the minimum and the initial heap size of the heap. If -Xms appears after -XX:InitialHeapSize on the command line, then the initial heap size gets set to the value specified with -Xms. |
-XX:InitialRAMPercentage=percent |
Sets the initial amount of memory that the JVM will use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 1.5625 percent.The following example shows how to set the percentage of the initial amount of memory used for the Java heap: -XX:InitialRAMPercentage=5 |
| -XX:InitialSurvivorRatio=ratio | Sets the initial survivor space ratio used by the throughput garbage collector (which is enabled by the -XX:+UseParallelGC and/or -XX:+UseParallelOldGC options). Adaptive sizing is enabled by default with the throughput garbage collector by using the -XX:+UseParallelGC and -XX:+UseParallelOldGC options, and survivor space is resized according to the application behavior, starting with the initial value. If adaptive sizing is disabled (using the -XX:-UseAdaptiveSizePolicy option), then the -XX:SurvivorRatio option should be used to set the size of the survivor space for the entire execution of the application.The following formula can be used to calculate the initial size of survivor space (S) based on the size of the young generation (Y), and the initial survivor space ratio (R): S=Y/(R+2) The 2 in the equation denotes two survivor spaces. The larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size. By default, the initial survivor space ratio is set to 8. If the default value for the young generation space size is used (2 MB), the initial size of the survivor space will be 0.2 MB. The following example shows how to set the initial survivor space ratio to 4: -XX:InitialSurvivorRatio=4 |
| -XX:InitiatingHeapOccupancyPercent=percent | Sets the percentage of the heap occupancy (0 to 100) at which to start a concurrent GC cycle. It is used by garbage collectors that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (for example, the G1 garbage collector). By default, the initiating value is set to 45%. A value of 0 implies nonstop GC cycles. The following example shows how to set the initiating heap occupancy to 75%: -XX:InitiatingHeapOccupancyPercent=75 |
| -XX:MaxGCPauseMillis=time | Sets a target for the maximum GC pause time (in milliseconds). This is a soft goal, and the JVM will make its best effort to achieve it. By default, there is no maximum pause time value. The following example shows how to set the maximum target pause time to 500 ms: -XX:MaxGCPauseMillis=500 |
| -XX:MaxHeapSize=size | Sets the maximum size (in byes) of the memory allocation pool. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value. See the section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at [http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html](http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html).The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units: -XX:MaxHeapSize=83886080 -XX:MaxHeapSize=81920k -XX:MaxHeapSize=80m On Oracle Solaris 7 and Oracle Solaris 8 SPARC platforms, the upper limit for this value is approximately 4,000 MB minus overhead amounts. On Oracle Solaris 2.6 and x86 platforms, the upper limit is approximately 2,000 MB minus overhead amounts. On Linux platforms, the upper limit is approximately 2,000 MB minus overhead amounts. The -XX:MaxHeapSize option is equivalent to -Xmx. |
| -XX:MaxHeapFreeRatio=percent | Sets the maximum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space expands above this value, then the heap will be shrunk. By default, this value is set to 70%. The following example shows how to set the maximum free heap ratio to 75%: -XX:MaxHeapFreeRatio=75 |
| -XX:MaxMetaspaceSize=size | Sets the maximum amount of native memory that can be allocated for class metadata. By default, the size is not limited. The amount of metadata for an application depends on the application itself, other running applications, and the amount of memory available on the system. The following example shows how to set the maximum class metadata size to 256 MB: -XX:MaxMetaspaceSize=256m |
| -XX:MaxNewSize=size | Sets the maximum size (in bytes) of the heap for the young generation (nursery). The default value is set ergonomically. |
| -XX:MaxRAMPercentage=percent | Sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 25 percent.Specifying this option disables automatic use of compressed oops if the combined result of this and other options influencing the maximum amount of memory is larger than the range of memory addressable by compressed oops. See -XX:UseCompressedOops for further information about compressed oops.The following example shows how to set the percentage of the maximum amount of memory used for the Java heap: -XX:MaxRAMPercentage=75 |
| -XX:MaxTenuringThreshold=threshold | Sets the maximum tenuring threshold for use in adaptive GC sizing. The largest value is 15. The default value is 15 for the parallel (throughput) collector, and 6 for the CMS collector. The following example shows how to set the maximum tenuring threshold to 10: -XX:MaxTenuringThreshold=10 |
| -XX:MetaspaceSize=size | Sets the size of the allocated class metadata space that will trigger a garbage collection the first time it is exceeded. This threshold for a garbage collection is increased or decreased depending on the amount of metadata used. The default size depends on the platform. |
| -XX:MinHeapFreeRatio=percent | Sets the minimum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space falls below this value, then the heap will be expanded. By default, this value is set to 40%. The following example shows how to set the minimum free heap ratio to 25%: -XX:MinHeapFreeRatio=25 |
| -XX:MinRAMPercentage=percent | Sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option for small heaps. A small heap is a heap of approximately 125 MB. The default value is 50 percent.The following example shows how to set the percentage of the maximum amount of memory used for the Java heap for small heaps: -XX:MinRAMPercentage=75 |
| -XX:NewRatio=ratio | Sets the ratio between young and old generation sizes. By default, this option is set to 2. The following example shows how to set the young/old ratio to 1: -XX:NewRatio=1 |
| -XX:NewSize=size | Sets the initial size (in bytes) of the heap for the young generation (nursery). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes.The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too low, then a large number of minor GCs will be performed. If the size is too high, then only full GCs will be performed, which can take a long time to complete. Oracle recommends that you keep the size for the young generation between a half and a quarter of the overall heap size. The following examples show how to set the initial size of young generation to 256 MB using various units: -XX:NewSize=256m -XX:NewSize=262144k -XX:NewSize=268435456 The -XX:NewSize option is equivalent to -Xmn. |
| -XX:ParallelGCThreads=threads | Sets the number of threads used for parallel garbage collection in the young and old generations. The default value depends on the number of CPUs available to the JVM. For example, to set the number of threads for parallel GC to 2, specify the following option: -XX:ParallelGCThreads=2 |
-XX:+ParallelRefProcEnabled |
Enables parallel reference processing. By default, this option is disabled. |
| -XX:+PrintAdaptiveSizePolicy | Enables printing of information about adaptive generation sizing. By default, this option is disabled. |
| -XX:+PrintGC | Enables printing of messages at every GC. By default, this option is disabled. |
| -XX:+PrintGCApplicationConcurrentTime | Enables printing of how much time elapsed since the last pause (for example, a GC pause). By default, this option is disabled. |
| -XX:+PrintGCApplicationStoppedTime | Enables printing of how much time the pause (for example, a GC pause) lasted. By default, this option is disabled. |
| -XX:+PrintGCDateStamps | Enables printing of a date stamp at every GC. By default, this option is disabled. |
| -XX:+PrintGCDetails | Enables printing of detailed messages at every GC. By default, this option is disabled. |
| -XX:+PrintGCTaskTimeStamps | Enables printing of time stamps for every individual GC worker thread task. By default, this option is disabled. |
| -XX:+PrintGCTimeStamps | Enables printing of time stamps at every GC. By default, this option is disabled. |
| -XX:+PrintStringDeduplicationStatistics | Prints detailed deduplication statistics. By default, this option is disabled. See the -XX:+UseStringDeduplication option. |
| -XX:+PrintTenuringDistribution | Enables printing of tenuring age information. The following is an example of the output: Desired survivor size 48286924 bytes, new threshold 10 (max 10) - age 1: 28992024 bytes, 28992024 total - age 2: 1366864 bytes, 30358888 total - age 3: 1425912 bytes, 31784800 total ... Age 1 objects are the youngest survivors (they were created after the previous scavenge, survived the latest scavenge, and moved from eden to survivor space). Age 2 objects have survived two scavenges (during the second scavenge they were copied from one survivor space to the next). And so on. In the preceding example, 28 992 024 bytes survived one scavenge and were copied from eden to survivor space, 1 366 864 bytes are occupied by age 2 objects, etc. The third value in each row is the cumulative size of objects of age n or less. By default, this option is disabled. |
| -XX:+ScavengeBeforeFullGC | Enables GC of the young generation before each full GC. This option is enabled by default. Oracle recommends that you do not disable it, because scavenging the young generation before a full GC can reduce the number of objects reachable from the old generation space into the young generation space. To disable GC of the young generation before each full GC, specify -XX:-ScavengeBeforeFullGC. |
| -XX:SoftRefLRUPolicyMSPerMB=time | Sets the amount of time (in milliseconds) a softly reachable object is kept active on the heap after the last time it was referenced. The default value is one second of lifetime per free megabyte in the heap. The -XX:SoftRefLRUPolicyMSPerMB option accepts integer values representing milliseconds per one megabyte of the current heap size (for Java HotSpot Client VM) or the maximum possible heap size (for Java HotSpot Server VM). This difference means that the Client VM tends to flush soft references rather than grow the heap, whereas the Server VM tends to grow the heap rather than flush soft references. In the latter case, the value of the -Xmx option has a significant effect on how quickly soft references are garbage collected.The following example shows how to set the value to 2.5 seconds: -XX:SoftRefLRUPolicyMSPerMB=2500 |
| -XX:StringDeduplicationAgeThreshold=threshold | String objects reaching the specified age are considered candidates for deduplication. An object's age is a measure of how many times it has survived garbage collection. This is sometimes referred to as tenuring; see the -XX:+PrintTenuringDistribution option. Note that String objects that are promoted to an old heap region before this age has been reached are always considered candidates for deduplication. The default value for this option is 3. See the -XX:+UseStringDeduplication option. |
| -XX:SurvivorRatio=ratio | Sets the ratio between eden space size and survivor space size. By default, this option is set to 8. The following example shows how to set the eden/survivor space ratio to 4: -XX:SurvivorRatio=4 |
| -XX:TargetSurvivorRatio=percent | Sets the desired percentage of survivor space (0 to 100) used after young garbage collection. By default, this option is set to 50%. The following example shows how to set the target survivor space ratio to 30%: -XX:TargetSurvivorRatio=30 |
| -XX:TLABSize=size | Sets the initial size (in bytes) of a thread-local allocation buffer (TLAB). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. If this option is set to 0, then the JVM chooses the initial size automatically.The following example shows how to set the initial TLAB size to 512 KB: -XX:TLABSize=512k |
| -XX:+UseAdaptiveSizePolicy | Enables the use of adaptive generation sizing. This option is enabled by default. To disable adaptive generation sizing, specify -XX:-UseAdaptiveSizePolicy and set the size of the memory allocation pool explicitly (see the -XX:SurvivorRatio option). |
| -XX:+UseCMSInitiatingOccupancyOnly | Enables the use of the occupancy value as the only criterion for initiating the CMS collector. By default, this option is disabled and other criteria may be used. |
-XX:+UseConcMarkSweepGC |
Enables the use of the CMS garbage collector for the old generation. Oracle recommends that you use the CMS garbage collector when application latency requirements cannot be met by the throughput (-XX:+UseParallelGC) garbage collector. The G1 garbage collector (-XX:+UseG1GC) is another alternative.By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. When this option is enabled, the -XX:+UseParNewGC option is automatically set and you should not disable it, because the following combination of options has been deprecated in JDK 8: -XX:+UseConcMarkSweepGC -XX:-UseParNewGC. |
| -XX:+UseG1GC | Enables the use of the garbage-first (G1) garbage collector. It is a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM. It meets GC pause time goals with high probability, while maintaining good throughput. The G1 collector is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements (stable and predictable pause time below 0.5 seconds). By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. |
| -XX:+UseGCOverheadLimit | Enables the use of a policy that limits the proportion of time spent by the JVM on GC before an OutOfMemoryError exception is thrown. This option is enabled, by default and the parallel GC will throw an OutOfMemoryError if more than 98% of the total time is spent on garbage collection and less than 2% of the heap is recovered. When the heap is small, this feature can be used to prevent applications from running for long periods of time with little or no progress. To disable this option, specify -XX:-UseGCOverheadLimit. |
| -XX:+UseNUMA | Enables performance optimization of an application on a machine with nonuniform memory architecture (NUMA) by increasing the application's use of lower latency memory. By default, this option is disabled and no optimization for NUMA is made. The option is only available when the parallel garbage collector is used (-XX:+UseParallelGC). |
| -XX:+UseParallelGC | Enables the use of the parallel scavenge garbage collector (also known as the throughput collector) to improve the performance of your application by leveraging multiple processors. By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. If it is enabled, then the -XX:+UseParallelOldGC option is automatically enabled, unless you explicitly disable it. |
| -XX:+UseParallelOldGC | Enables the use of the parallel garbage collector for full GCs. By default, this option is disabled. Enabling it automatically enables the -XX:+UseParallelGC option. |
| -XX:+UseParNewGC | Enables the use of parallel threads for collection in the young generation. By default, this option is disabled. It is automatically enabled when you set the -XX:+UseConcMarkSweepGC option. Using the -XX:+UseParNewGC option without the -XX:+UseConcMarkSweepGC option was deprecated in JDK 8. |
| -XX:+UseSerialGC | Enables the use of the serial garbage collector. This is generally the best choice for small and simple applications that do not require any special functionality from garbage collection. By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. |
| -XX:+UseSHM | On Linux, enables the JVM to use shared memory to setup large pages. For more information, see "Large Pages". |
| -XX:+UseStringDeduplication | Enables string deduplication. By default, this option is disabled. To use this option, you must enable the garbage-first (G1) garbage collector. See the -XX:+UseG1GC option.String deduplication reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that many String objects are identical. Instead of each String object pointing to its own character array, identical String objects can point to and share the same character array. |
-XX:+UseTLAB |
Enables the use of thread-local allocation blocks (TLABs) in the young generation space. This option is enabled by default. To disable the use of TLABs, specify -XX:-UseTLAB. |
1.3.6 过期和移除的选项
| 选项 | 说明 |
|---|---|
| -Xincgc | Enables incremental garbage collection. This option was deprecated in JDK 8 with no replacement. |
| -Xrun_libname_ | Loads the specified debugging/profiling library. This option was superseded by the -agentlib option. |
| -XX:CMSIncrementalDutyCycle=percent | Sets the percentage of time (0 to 100) between minor collections that the concurrent collector is allowed to run. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option. |
| -XX:CMSIncrementalDutyCycleMin=percent | Sets the percentage of time (0 to 100) between minor collections that is the lower bound for the duty cycle when -XX:+CMSIncrementalPacing is enabled. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option. |
| -XX:+CMSIncrementalMode | Enables the incremental mode for the CMS collector. This option was deprecated in JDK 8 with no replacement, along with other options that start with CMSIncremental. |
-XX:CMSIncrementalOffset=percent |
Sets the percentage of time (0 to 100) by which the incremental mode duty cycle is shifted to the right within the period between minor collections. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option. |
| -XX:+CMSIncrementalPacing | Enables automatic adjustment of the incremental mode duty cycle based on statistics collected while the JVM is running. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option. |
-XX:CMSIncrementalSafetyFactor=percent |
Sets the percentage of time (0 to 100) used to add conservatism when computing the duty cycle. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option. |
| -XX:CMSInitiatingPermOccupancyFraction=percent |
Sets the percentage of the permanent generation occupancy (0 to 100) at which to start a GC. This option was deprecated in JDK 8 with no replacement. |
| -XX:MaxPermSize=size | Sets the maximum permanent generation space size (in bytes). This option was deprecated in JDK 8, and superseded by the -XX:MaxMetaspaceSize option. |
| -XX:PermSize=size | Sets the space (in bytes) allocated to the permanent generation that triggers a garbage collection if it is exceeded. This option was deprecated un JDK 8, and superseded by the -XX:MetaspaceSize option. |
| -XX:+UseSplitVerifier | Enables splitting of the verification process. By default, this option was enabled in the previous releases, and verification was split into two phases: type referencing (performed by the compiler) and type checking (performed by the JVM runtime). This option was deprecated in JDK 8, and verification is now split by default without a way to disable it. |
| -XX:+UseStringCache | Enables caching of commonly allocated strings. This option was removed from JDK 8 with no replacement. |