参考：
http://stackoverflow.com/questions/344203/maximum-number-of-threads-per-process-in-linux
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理论上相关限制的配置文件为：
/proc/sys/kernel/threads-max
/proc/sys/vm/max_map_count??
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实际上最大线程数计算为：
number of threads = total virtual memory / (stack size*1024*1024)?
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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
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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
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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。?
