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From Jeroen Van den Horn <>
Subject [users@httpd] Intermittent stalls
Date Mon, 26 Nov 2012 07:26:41 GMT
I hope somebody can help me with an issue that's kept me busy for quite
some days now.

We are using LVS in combination with Apache and mod-proxy to serve content
from a large number of back-end servers (primarily using mod_proxy_balancer
and mod_proxy_ajp).

All runs smoothly until one of the back-end servers start behaving
eratically (i.e. takes a long time to respond). When this occurs the apache
server seems to stall - that is, after some time it seems to get stuck
waiting for the request to time-out. In the intermediate time though it is
still serving requests just fine. I've tried the worker, prefork and event
MPM and all experience this same behavior.

Could it be that something like this is happening:

The 'master' apache process that dispatches incoming connections to the
underlying servers allocates a 'slot' to process a request (for the sake of
the argument, call this slot #10). Further requests are received and
allocated to higher numbered slots. This goes fine until the list wraps and
at a certain point we arrive back at slot #10. Now for some obscure reason
apache appears to somehow sit-and-wait until this connection either
times-out or deliveres the data. As soon as than happens everything just
runs fine again, but during this intermittent time the server just appears
to sit and do nothing (and appears stalled). At this moment we cannot even
serve local content (which - as a side effect causes the LVS director to
box the node off-line).

Has anybody ever seen Apache behaving like this? Could it be something to
do with some module that we use? We use mod_perl quite extensively (and
that's where my gut feelings point to but before rewrite all our code in a
native mod_xxx module I would like to have just a little more evidence).

The apache version we are running is:

Server version: Apache/2.2.17 (Ubuntu)
Server built:   Feb 14 2012 19:59:03

Any hints on how to pursue this problem further are much appreciated.


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