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From Ruediger Pluem <>
Subject Re: 2.0.54 unstable, requests time-out, NO warnings in logs
Date Mon, 01 Oct 2007 08:23:08 GMT

On 10/01/2007 08:32 AM, Alec Matusis wrote:
> 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 

Have you checked without the MaxMemFree setting?
Why do you use MaxMemFree with such a small value at all?



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