memcached 1.6.17
-p, --port=<num>          小写p，memcached监听的tcp端口。（默认端口为11211）
-U, --udp-port=<num>      大写U memcached监听的UDP端口值，（默认端口为0, 关闭）（老版1.4默认开11211）
-s, --unix-socket=<file>  小写S。unix socket监听的socket路径。进程间通信(关闭网络支持)
-a, --unix-mask=<mask>    小写A。unix socket的权限位信息(访问掩码， 八进制) (默认: 700)
-A, --enable-shutdown     大写A。允许客户端使用shutdown命令。默认是不允许的。该选项将允许客户端的shutdown命令关闭memcached
-l, --listen=<addr>       小写L。memcached绑定的ip地址。如果不设置这个选项，那么memcached将使用INADDR_ANY。如果想指定多个IP地址，那么该选项的参数可以由多个ip组成，ip之间用逗号分隔。也可以多次使用这个选项，此时端口应该尾随ip而不是单独用-p选项指定。例如-l 127.0.0.1:8888,192.168.1.112:9999 或者 -l 127.0.0.1:8888 -l 192.168.1.112:9999
-d, --daemon              以守护进程的形式运行Memcached
-r, --enable-coredumps    将core文件大小设置为不受限制
-u, --user=<user>         小写u。当以root用户启动memcached的时候需要指定memcached的所属用户，其他用户启动memcached不需要此选项
-m, --memory-limit=<num>  小写m。memcached能够使用的最大内存值，默认是64MB。参数单位为MB。
-M, --disable-evictions   大写M。内存用尽时报错，而不是淘汰数据。（默认情况下，当memcached的内存使用完后，将进行LRU机制淘汰item以腾出空间。如果使用本选项那么将关闭LRU功能。当然关闭LRU不代表不能存储新数据。如果memcached里面存有过期失效的item，那么就可以存储新数据。否则将无法存储。）
-c, --conn-limit=<num>    小写c。最多允许多少个客户端同时在线(这个值不等价于listen函数的第二个参数)，该选项和后面的b选项有所不同。 默认值为1024个
-k, --lock-memory         小写k。将memcached使用到的内存锁定在内存中，不准OS把memcached的内存移动到虚拟内存。因为当OS把memcached的内存移动到虚拟内存可能会导致页错误，降低memcached的响应时间
-v, --verbose             小写v。输出memcached运行时的错误和警告日志。-v -vv -vvv输出的信息依次增加。 
-vv                       very verbose (also print client commands/responses)
-vvv                      extremely verbose (internal state transitions)
-h, --help                print this help and exit
-i, --license             显示memcached和libevent的版权信息
-V, --version             print version and exit
-P, --pidfile=<file>      大写p。该选项的参数指明memcached的pid保存文件。要和-d选项配合使用。注意运行的用户是否有权限写对应的文件
-f, --slab-growth-factor=<num>   item的扩容因子。默认值为1.25。该选项的参数值可以是小数但必须大于1.0
-n, --slab-min-size=<bytes>      设置最小的item能存储多少字节的数据。
-L, --enable-largepages          如果OS允许的话，那么向OS申请更大的内存页。OS的默认内存页为4KB。大的内存页可以有效降低页表的大小，提高效率。此选项会使得memcached预先先OS全部所需的申请内存。当然这些内存尽量是用大内存页分配的
-D <char>                 指定key前缀和ID的分隔符。用于stats报告状态信息，默认的分隔符为冒号":"。指定该选项会自动打开报告收集。也可以在启动memcached后，客户端使用stats detail on命令开启，。
-t, --threads=<num>       该选项的参数用于指定worker线程的个数，不建议超过64个。如果不设置该选项默认有4个线程
-R, --max-reqs-per-event   worker线程连续为某个客户端执行命令的最大命令数。控制这个每个连接的请求数，可避免阻塞（默认20）
-C, --disable-cas         大写C。memcached默认是使用CAS的，本选项是禁用CAS
-b, --listen-backlog=<num>  listen函数的第二个参数。该选项的参数赋值给settings.backlog。如果不设置该选项，那么默认为1024。该选项和前面的c选项有所不同
-B, --protocol=<name>     memcached支持文本协议和二进制协议。该选项的参数用于指定使用的协议。默认情况下是根据客户端的命令而自动判断(也叫协商)，参数只能取auto、binary、ascii这三个字符串值。
-I, --max-item-size=<num> 大写i。slab分配器中，每一个页的大小。这个选项的参数是一个数值表示页的大小。默认单位是B也可以在数值后面带K或者M(大小写都行)，表示KB和MB。页的大小小于1KB或者大于128MB都是不允许的。不推荐使用该选项。
-F, --disable-flush-all   禁止客户端的flush_all命令。默认是允许客户端的flush_all命令的
-X, --disable-dumping     禁用 stats cachedump 和 lru_crawler metadump
-W  --disable-watch       禁用watch 命令
-Y, --auth-file=<file>    （实验）使用ascii协议进行认证。 格式  用户名:密码\n 用户:密码
-e, --memory-file=<file>  （实验）指定文件来映射内存，只有在ram磁盘或者持久内存映射中使用。使缓存支持可重启
-o, --extended            逗号分割的扩展参数，大部分参数可以加上’no_’前缀来关闭
   - maxconns_fast:       如果连接数超过了最大同时在线数(由-c选项指定)，立即关闭新连接上的客户端
   - hashpower:           哈希表的长度是2^n。这个值就是n的初始值。可以在启动memcached的时候通过-o hashpower设置。                           设置的值要在[12, 64]之间。如果不设置，该值为0。哈希表的幂将取默认值16 (HASHPOWER_DEFAULT = 16)
                          set based on "STAT hash_power_level"
   - tail_repair_time:    用于检测是否有item被已死线程所引用。一般不会出现这种情况，所以默认不开启这种检测。如果需要开启这种检测，那么需要使用本选项。本选项需要一个参数，参数值必须不小于10。

                          disabled by default; very dangerous option.
   - hash_algorithm:      用于指定哈希算法。该选项必须带有参数。并且参数只能是字符串jenkins或者murmur3 options: jenkins, murmur3, xxh3
   - no_lru_crawler:      禁用LRU爬虫线程。该选项不需要参数。

   - lru_crawler_sleep:   LRU爬虫线程工作时的休眠间隔。本选项需要一个参数作为休眠时间，单位为微秒，取值范围是[0, 1000000]。
   - lru_crawler_tocrawl: LRU爬虫检查每条LRU队列中的多少个item。该选项带有一个参数。

   - read_buf_mem_limit:  每个连接读写的缓冲大小，单位mb。除非连接非常多（2w+），不然			不要调整。默认为0， 不限制大小。

   - no_lru_maintainer:   关闭新的 LRU系统和后台线程 （改进的分段LRU算法（Segmented	LRU））
   - hot_lru_pct:         LRU热数据的比例，默认20. 需要开启lru_maintainer
   - warm_lru_pct:        LRU暖数据的比例，默认40. 需要开启lru_maintainer
   - hot_max_factor:      HOT 队列 tail age 大于 COLD 队列 tail age *该值
   - warm_max_factor:     WARM 队列 tail age 大于 COLD 队列 tail age *该值
   - temporary_ttl:       小于该值的 item 进入 TEMP LRU，且除非 item 过期或删除，否则不会离开该队列，不会被剔除
   - idle_timeout:        空闲线程的超时主动断开时间。默认是0， 不会超时（1.4.27）
   - slab_chunk_max:      最大的slab 的 chunk大小。默认512K
   - watcher_logbuf_size: watch命令启用流式日志时，日志的上限。提高可以避免日志满了丢失
   - worker_logbuf_size:  每个worker的log输出限制，被后台线程读取，并写到watchers
   - track_sizes:         显示每个slab 所用的大小。 这样可以直接用stats sizes命令去看分配		的数据大小（不然要用stats sizes_enable 来打开）
   - no_hashexpand:       禁止hash扩容（危险）
   - modern:              已经打开了的特性，以后会设置成默认

   - no_modern:           使用老版的特性。用于禁用 slab_reassign、slab_automove、lru_crawler、	lru_maintainer、maxconns_fast 命令的别名。No modern 还将 hash_algorithm 设置为 	jenkins，并允许内联 ASCII VALUE。适用于 memcached 1.5 版和更高版本。要还原为 	modern（目前是默认值），您必须重新启动。


   - External storage       外部存储相关参数 (see: https://memcached.org/extstore)
   - ext_path:            将数据存储至内存之外其他缓存介质。比如 -o ext_path=/path/to/a/datafile:5G
   - ext_page_size:       size in megabytes of storage pages. (default: 64)
   - ext_wbuf_size:       size in megabytes of page write buffers. (default: 4)
   - ext_threads:         number of IO threads to run. (default: 1)
   - ext_item_size:       store items larger than this (bytes, default 512)
   - ext_item_age:        store items idle at least this long (seconds, default: no age limit)
   - ext_low_ttl:         consider TTLs lower than this specially (default: 0)
   - ext_drop_unread:     don't re-write unread values during compaction (default: disabled)
   - ext_recache_rate:    recache an item every N accesses (default: 2000)
   - ext_compact_under:   compact when fewer than this many free pages
                          (default: 1/4th of the assigned storage)
   - ext_drop_under:      drop COLD items when fewer than this many free pages
                          (default: 1/4th of the assigned storage)
   - ext_max_frag:        max page fragmentation to tolerate (default: 0.80)
   - ext_max_sleep:       max sleep time of background threads in us (default: 1000000)
   - slab_automove_freeratio: ratio of memory to hold free as buffer.
                          (see doc/storage.txt for more info, default: 0.010)
-N, --napi_ids            number of napi ids. see doc/napi_ids.txt for more details

memcached 1.6.17
-p, --port=<num>          TCP port to listen on (default: 11211)
-U, --udp-port=<num>      UDP port to listen on (default: 0, off)
-s, --unix-socket=<file>  UNIX socket to listen on (disables network support)
-a, --unix-mask=<mask>    access mask for UNIX socket, in octal (default: 700)
-A, --enable-shutdown     enable ascii "shutdown" command
-l, --listen=<addr>       interface to listen on (default: INADDR_ANY)
-d, --daemon              run as a daemon
-r, --enable-coredumps    maximize core file limit
-u, --user=<user>         assume identity of <username> (only when run as root)
-m, --memory-limit=<num>  item memory in megabytes (default: 64)
-M, --disable-evictions   return error on memory exhausted instead of evicting
-c, --conn-limit=<num>    max simultaneous connections (default: 1024)
-k, --lock-memory         lock down all paged memory
-v, --verbose             verbose (print errors/warnings while in event loop)
-vv                       very verbose (also print client commands/responses)
-vvv                      extremely verbose (internal state transitions)
-h, --help                print this help and exit
-i, --license             print memcached and libevent license
-V, --version             print version and exit
-P, --pidfile=<file>      save PID in <file>, only used with -d option
-f, --slab-growth-factor=<num> chunk size growth factor (default: 1.25)
-n, --slab-min-size=<bytes> min space used for key+value+flags (default: 48)
-L, --enable-largepages  try to use large memory pages (if available)
-D <char>     Use <char> as the delimiter between key prefixes and IDs.
              This is used for per-prefix stats reporting. The default is
              ":" (colon). If this option is specified, stats collection
              is turned on automatically; if not, then it may be turned on
              by sending the "stats detail on" command to the server.
-t, --threads=<num>       number of threads to use (default: 4)
-R, --max-reqs-per-event  maximum number of requests per event, limits the
                          requests processed per connection to prevent
                          starvation (default: 20)
-C, --disable-cas         disable use of CAS
-b, --listen-backlog=<num> set the backlog queue limit (default: 1024)
-B, --protocol=<name>     protocol - one of ascii, binary, or auto (default: auto-negotiate)
-I, --max-item-size=<num> adjusts max item size
                          (default: 1m, min: 1k, max: 1024m)
-F, --disable-flush-all   disable flush_all command
-X, --disable-dumping     disable stats cachedump and lru_crawler metadump
-W  --disable-watch       disable watch commands (live logging)
-Y, --auth-file=<file>    (EXPERIMENTAL) enable ASCII protocol authentication. format:
                          user:pass\nuser2:pass2\n
-e, --memory-file=<file>  (EXPERIMENTAL) mmap a file for item memory.
                          use only in ram disks or persistent memory mounts!
                          enables restartable cache (stop with SIGUSR1)
-o, --extended            comma separated list of extended options
                          most options have a 'no_' prefix to disable
   - maxconns_fast:       immediately close new connections after limit (default: enabled)
   - hashpower:           an integer multiplier for how large the hash
                          table should be. normally grows at runtime. (default starts at: 0)
                          set based on "STAT hash_power_level"
   - tail_repair_time:    time in seconds for how long to wait before
                          forcefully killing LRU tail item.
                          disabled by default; very dangerous option.
   - hash_algorithm:      the hash table algorithm
                          default is murmur3 hash. options: jenkins, murmur3, xxh3
   - no_lru_crawler:      disable LRU Crawler background thread.
   - lru_crawler_sleep:   microseconds to sleep between items
                          default is 100.
   - lru_crawler_tocrawl: max items to crawl per slab per run
                          default is 0 (unlimited)
   - read_buf_mem_limit:  limit in megabytes for connection read/response buffers.
                          do not adjust unless you have high (20k+) conn. limits.
                          0 means unlimited (default: 0)
   - no_lru_maintainer:   disable new LRU system + background thread.
   - hot_lru_pct:         pct of slab memory to reserve for hot lru.
                          (requires lru_maintainer, default pct: 20)
   - warm_lru_pct:        pct of slab memory to reserve for warm lru.
                          (requires lru_maintainer, default pct: 40)
   - hot_max_factor:      items idle > cold lru age * drop from hot lru. (default: 0.20)
   - warm_max_factor:     items idle > cold lru age * this drop from warm. (default: 2.00)
   - temporary_ttl:       TTL's below get separate LRU, can't be evicted.
                          (requires lru_maintainer, default: 61)
   - idle_timeout:        timeout for idle connections. (default: 0, no timeout)
   - slab_chunk_max:      (EXPERIMENTAL) maximum slab size in kilobytes. use extreme care. (default: 512)
   - watcher_logbuf_size: size in kilobytes of per-watcher write buffer. (default: 256)
   - worker_logbuf_size:  size in kilobytes of per-worker-thread buffer
                          read by background thread, then written to watchers. (default: 64)
   - track_sizes:         enable dynamic reports for 'stats sizes' command.
   - no_hashexpand:       disables hash table expansion (dangerous)
   - modern:              enables options which will be default in future.
                          currently: nothing
   - no_modern:           uses defaults of previous major version (1.4.x)

   - External storage (ext_*) related options (see: https://memcached.org/extstore)
   - ext_path:            file to write to for external storage.
                          ie: ext_path=/mnt/d1/extstore:1G
   - ext_page_size:       size in megabytes of storage pages. (default: 64)
   - ext_wbuf_size:       size in megabytes of page write buffers. (default: 4)
   - ext_threads:         number of IO threads to run. (default: 1)
   - ext_item_size:       store items larger than this (bytes, default 512)
   - ext_item_age:        store items idle at least this long (seconds, default: no age limit)
   - ext_low_ttl:         consider TTLs lower than this specially (default: 0)
   - ext_drop_unread:     don't re-write unread values during compaction (default: disabled)
   - ext_recache_rate:    recache an item every N accesses (default: 2000)
   - ext_compact_under:   compact when fewer than this many free pages
                          (default: 1/4th of the assigned storage)
   - ext_drop_under:      drop COLD items when fewer than this many free pages
                          (default: 1/4th of the assigned storage)
   - ext_max_frag:        max page fragmentation to tolerate (default: 0.80)
   - ext_max_sleep:       max sleep time of background threads in us (default: 1000000)
   - slab_automove_freeratio: ratio of memory to hold free as buffer.
                          (see doc/storage.txt for more info, default: 0.010)
-N, --napi_ids            number of napi ids. see doc/napi_ids.txt for more details