最近，公司线上tomcat经常无缘无辜宕机，总结了一下定位问题的方法，仅供参考：
报错信息：
Maximum number of threads (200) created for connector with address null and port 9443
# There is insufficient memory for the Java Runtime Environment to continue.
# Cannot create GC thread. Out of system resources.
一、查看当前用户线程和文件句柄数是否超出限制
（1）显示当前用户进程限制：ulimit -a
显示结果：
core file size (blocks, -c) 0
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 256612
max locked memory (kbytes, -l) 64
max memory size (kbytes, -m) unlimited
open files (-n) 102400
pipe size (512 bytes, -p) 8
POSIX message queues (bytes, -q) 819200
real-time priority (-r) 0
stack size (kbytes, -s) 10240
cpu time (seconds, -t) unlimited
max user processes (-u) 1024
virtual memory (kbytes, -v) unlimited
file locks (-x) unlimited
（2）修改所有 linux 用户的环境变量文件：
vi /etc/profile
ulimit -u 10000
ulimit -n 4096
保存后运行#source /etc/profile 使其生效
二、查看当前端口号进程信息和GC使用情况
（1）显示端口的PID：lsof -i:端口
示例：lsof -i:7074
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
java 3195 ligang 34u IPv4 37416693 0t0 TCP *:7074 (LISTEN)
（2）gc信息统计：jstat -gcutil PID
示例：jstat -gcutil 3195
S0 S1 E O P YGC YGCT FGC FGCT GCT
12.63 0.00 52.03 78.63 99.13 4148 24.274 200 40.246 64.520
（3）输出线程数：ps -mp PID -o THREAD,tid,time | wc -l
示例：ps -mp 3195 -o THREAD,tid,time | wc -l
43
三、查看进程内存使用情况及定位到对应程序
（1）内存使用情况：top -p PID
示例：top 3195
top - 15:29:27 up 25 days, 20:05, 2 users, load average: 0.01, 0.05, 0.01
Tasks: 1 total, 0 running, 1 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.0%us, 0.1%sy, 0.0%ni, 99.8%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 8058868k total, 6821684k used, 1237184k free, 181936k buffers
Swap: 2097144k total, 492300k used, 1604844k free, 1897320k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3195 ligang 20 0 4862m 196m 10m S 0.0 2.5 7:57.48 java
（2）找到该进程后，如何定位具体线程或代码呢，首先显示线程列表,并按照CPU占用高的线程排序：
ps -mp PID -o THREAD,tid,time | sort -rn | head -10
示例：ps -mp PID -o THREAD,tid,time | sort -rn | head -10
USER %CPU PRI SCNT WCHAN USER SYSTEM TID TIME
ligang 0.6 - - - - - - 00:07:58
ligang 0.2 19 - futex_ - - 3270 00:02:49
ligang 0.0 19 - inet_c - - 3277 00:00:00
ligang 0.0 19 - inet_c - - 3273 00:00:00
ligang 0.0 19 - inet_c - - 3271 00:00:00
ligang 0.0 19 - inet_c - - 3203 00:00:05
ligang 0.0 19 - futex_ - - 7644 00:00:00
ligang 0.0 19 - futex_ - - 3420 00:00:00
ligang 0.0 19 - futex_ - - 3288 00:00:06
（3）将需要的线程ID转换为16进制格式：printf "%x\n" TID
示例：printf "%x\n" 3270
cc6
（4）最后打印线程的堆栈信息：jstack PID |grep cc6 -A 30
示例：
jstack 2633 |grep e18 -A 30
结果就可以看到哪段代码导致的问题...