Hello,
I am running nginx 0.7.67-3 on debian, now without php, mostly serving one empty html. It was working fine for several weeks, handling about 800 connections per second with 2 work processes. Few days ago it started to be very slow, during peak hours almost impossible to connect, but I don't see any bottleneck. I tried increasing the number of workers, using one 1 worker, using multi_accept - nothing helped. What I see on server is that I have more than 500 sockets in SYN_RECV state and nginx process seems to be doing nothing... strace shows this:

gettimeofday({1312807826, 920091}, NULL) = 0
epoll_wait(16, {}, 512, 500) = 0
gettimeofday({1312807827, 421795}, NULL) = 0
epoll_wait(16, {}, 512, 500) = 0
gettimeofday({1312807827, 923222}, NULL) = 0
epoll_wait(16, {}, 512, 500) = 0
gettimeofday({1312807828, 424994}, NULL) = 0
epoll_wait(16, {}, 512, 500) = 0
gettimeofday({1312807828, 926449}, NULL) = 0
epoll_wait(16, {}, 512, 500) = 0
gettimeofday({1312807829, 428064}, NULL) = 0
epoll_wait(16, {}, 512, 500) = 0

like if it had nothing to do - despite those waiting connections... I din't change anything on the server or nginx itself. Maybe the traffic is different (coming from different countries for example), but that shouldn't have any influence.
The traffic is being moved from this old server and domain to a new one. The bad thing is that today the slowness started also on the new server! There we have nginx 1.0.5 from debian dotdeb with php5-fpm. Traffic is to one php script which does one INSERT to the mysql (ad tracking). The new server has 6 GB RAM, 6-core Xeon CPU and is almost idle.

epoll_wait(6, {}, 512, 500) = 0
epoll_wait(6, {}, 512, 500) = 0
epoll_wait(6, {}, 512, 500) = 0
epoll_wait(6, {}, 512, 500) = 0
epoll_wait(6, {}, 512, 500) = 0

What could be the problem?