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From Alec Matusis <>
Subject 2.0.54 unstable, requests time-out, NO warnings in logs
Date Mon, 01 Oct 2007 06:32:05 GMT
We are running a busy Apache/2.0.54 server on 2.6.9 kernel, that suddenly becomes very slow-
requests either time out, or it takes 10-20sec to serve a 1K thumbnail. 
It is somewhat correlated with load spikes, but not perfectly (by looking at the bandwidth
graph, it never happens during the low bandwidth periods at night, but it does not coincide
with peaks of b/w) 

When we initially encountered an apache overload, it was always accompanied with 

[error] server reached MaxClients setting, consider raising the MaxClients setting 

in the apache error log. We also got 

kernel: possible SYN flooding on port 80. Sending cookies. 

in /var/log/messages system log. 

After that I raised MaxClients from 200 to 300. The problem initially disappeared, but after
our bandwidth grew a bit more, we got this behavior again. 
Now apache crashes (becomes very slow) silently, with no warning in apache error logs at all
(although we still get SYN flood message in the system log) 
When apache is this 'slow' regime, /server-status still shows available slots, i.e. MaxClients
is not reached. 

This is the relevant part of httpd.conf: 

ServerLimit 300 
# we are using prefork MPM 
StartServers 10 
MinSpareServers 5 
MaxSpareServers 20 
MaxClients 300 
MaxRequestsPerChild 10000 
MaxMemFree 2500 

The server has 4GB of physical RAM and 4GB of swap. During these apache “slowdowns", the
swap size is still 0 and vmstat shows no swapping at all. 
I suspect the problem may be in 

MaxMemFree 2500 

but then I would expect some kind of  “out of memory” errors in the logs? 

I am posting it on this list since I have not gotten a response in the users list, and I think
it's a bug for two reasons: 

1) When apache is in this slow "degraded" regime, I would expect a log message in the apache
error log, with an explanation why. 

3) If this is related to resource exhaustion, I would expect the server to recover from this
regime by itself when the load subsides, but this is not the case. Only apachectl start/stop
recovers the server.

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