使用 ulimit -a 命令，查看 max user processes 的输出，就是系统最大进程数
core file size (blocks, -c) unlimited data seg size (kbytes, -d) unlimited scheduling priority (-e) 0 file size (blocks, -f) unlimited pending signals (-i) 31216 max locked memory (kbytes, -l) 16384 max memory size (kbytes, -m) unlimited open files (-n) 1024 pipe size (512 bytes, -p) 8 POSIX message queues (bytes, -q) 819200 real-time priority (-r) 0 stack size (kbytes, -s) 8192 cpu time (seconds, -t) unlimited max user processes (-u) 31216 virtual memory (kbytes, -v) unlimited file locks (-x) unlimited
max user processes 的值默认是 # cat /proc/sys/kernel/threads-max的值/2,即系统线程数的一半