1 fcgi cannot work well but 5 fcgi process does.

[Miao Jiang]

I use nginx/0.6.31+fastcgi to deploy ruby on rails.
then I use ab to do 'ab -n1000 -c500' for the pressure test.
when it only run with 1 fastcgi process.
It works well with ab -n1000 -c1 to -c100
but in -c500,after about 800 request, the response suddenly returns very 
slowly even timeout. I use 'top' to see the process status, fcgi process 
isn't busy any more.

then I try add 4 more process, then it works well.
Can anyone could help me understand how it happens and how to setup 
nginx let it could finish -c500 test with only 1 fcgi process.

Thanks
Miao