There is Debian Squeeze servers:
1) Load Balancer - Haproxy 1.4.18, 16 cpu cores, 5-10 GB RAM (it's cloud service)
2) 4 Apache backends (10 GB RAM, 16 cpu cores)
When we start haproxy, site works well for about 2 minutes, then for about 30 seconds it hangs. (At this moment direct access to backends works well, thereis no slow database queries or any backend problems) After that situation repeats. Interesting observation: we tried to use nginx as balancer and we got the SAME behaviour.
When haproxy (nginx) hang, there is no any cpu or ram highload.
What can be reason of this behaviour?