参考：
http://stackoverflow.com/questions/344203/maximum-number-of-threads-per-process-in-linux
理论上相关限制的配置文件为：
/proc/sys/kernel/threads-max
/proc/sys/vm/max_map_count
实际上最大线程数计算为：
number of threads = total virtual memory / (stack size*1024*1024)
比如当前机器ulimit为：
socol@ubuntu:/etc/php5$ ulimit -a
core file size (blocks, -c) unlimited
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 23963
max locked memory (kbytes, -l) 64
max memory size (kbytes, -m) unlimited
open files (-n) 65535
pipe size (512 bytes, -p) 8
POSIX message queues (bytes, -q) 819200
real-time priority (-r) 0
stack size (kbytes, -s) 8192 (= 8k)
cpu time (seconds, -t) unlimited
max user processes (-u) 23963
virtual memory (kbytes, -v) unlimited
file locks (-x) unlimited
内存大小为：
socol@ubuntu:/etc/php5$ top
top - 12:51:21 up 1 day, 6:44, 2 users, load average: 0.02, 0.12, 0.17
Tasks: 189 total, 1 running, 185 sleeping, 0 stopped, 3 zombie
Cpu(s): 3.2%us, 1.3%sy, 0.0%ni, 95.5%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem:
3082008k total, 1606356k used, 1475652k free, 128260k buffers
Swap: 783356k total, 3512k used, 779844k free, 723588k cached
最大线程数为：
3082008k/(8*1024*1024) 约等于 376个线程，与实际测试结果相符。