Problem of load balance among php-cgi process

Hi, I've got a problem when running PHP with nginx, there are several php-cgi processes and only one is busy at one time, like this: 500 23868 0.0 0.0 209164 10948 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23871 0.0 0.0 209136 10684 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23872 0.0 0.0 209136 10732 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23873 0.0 0.0 209136 10712 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23874 0.0 0.0 209076 10576 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23875 0.0 0.0 209136 10976 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23876 0.0 0.0 209136 10796 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23877 0.0 0.0 209136 11068 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23878 0.0 0.0 209136 10708 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23879 0.0 0.0 209136 10960 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 23880 0.0 0.0 209136 10832 ? S 21:23 0:01 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf 500 27195 15.0 0.1 210196 12644 ? S 23:02 0:02 /usr/local/websrv/php/bin/php-cgi --fpm --fpm-config /usr/local/websrv/php/etc/php-fpm.conf The red one is busy, and other php-cgi processes is cpu free, cpu load is about 20% total, with lot of free memory, disk and network io status is fine, and mean time, response of HTTP request to this server is very slow. The cpu load of php-cgi processes return normal when I restart the php-fpm service (each process takes some cpu slice which means they are all working I think). But, after several minutes, the problem appears again and again. what's wrong with my php-cgi? PS: how do I debug or monitor the php program running status when it goes wrong? versions: php: 5.2.17 nginx: 5.8.55 OS: CentOS 5.6 64-bit Regards Sean