nginx request_time大于POST请求的实际php执行时间

Question

I'm running nginx + php-fpm.

nginx relevant configuration:
worker_processes auto;
worker_connections 1024;

php-fpm relevant configuration:

pm = dynamic
pm.max_children = 256
pm.start_servers = 32
pm.min_spare_servers = 8
pm.max_spare_servers = 32
pm.max_requests = 10000

from time to time (~5%) of POST requests become very slow - the request_time value in access log is about 2 seconds while it takes about 3-5 ms to php to handle the request. It's body length is about 200 bytes and response size is also pretty low - less than 1 kb.

Looks like miss configuration. Any suggestions which configuration should be tuned?

thanks.