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Date: Mon, 1 Oct 2007 08:54:43 +0200
From: Christian Folini <christian.folini@post.ch>
To: users@httpd.apache.org
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Subject: Re: [users@httpd] requests time-out under load, no warnings in logs

Hey Alec,

This looks like a tricky problem. Have you looked at the traffic
using tcpdump or ethereal when the machine hangs? You might
also try out mod_forensic or mod_security and add mod_security
internal timestamps to the access-log. Using these logs you will
get a clearer idea where it hangs.

How does your apache look memory-wise in top/htop? 
Are you sure it is not an application issue - even if it shows
serving static files. A reverse proxy setup for testing purposes
could seperate application and static file serving to
give you a clearer idea in this regard.

These are just a lot of ideas. I do not really know how to
solve your problem, but this would give you a clearer idea
of the problem.

I am not familiar with best practice values for MaxMemFree.

regs,

Christian



On Sun, Sep 30, 2007 at 03:54:59PM -0700, 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 10s to serve a 1K
> thumbnail. 
> It is usually correlated with load spikes, but not perfectly (by looking at
> the bandwidth graph)
> Then it requires a stop/start, and it becomes fast again.
> This has been happening before, and it was always accompanied with 
> 
> [error] server reached MaxClients setting, consider raising the MaxClients
> setting 
> 
> in the apache log.
> 
> We raised MaxClients    from 200 to 300, and now it happens with no warning
> in any logs at all, except with the usual
> 
> web10 kernel: possible SYN flooding on port 80. Sending cookies. in
> /var/log/messages (this has always happened when apache hung from overload).
> 
> What worries me is that there are no log messages in apache logs at all when
> this happens.
> 
> This is the relevant part of httpd.conf
> 
> ServerLimit 300
> # we are using prefork MPM
> <IfModule prefork.c>
> StartServers          10
> MinSpareServers       5
> MaxSpareServers       20
> MaxClients            300
> MaxRequestsPerChild   10000
> MaxMemFree            2560
> </IfModule>
> 
> The server has 4GB of RAM and 4GB of swap. When apache "crashes" i.e.
> becomes very slow, the swap size is still 0 and vmstat shows no swapping.
> I suspect the problem may be in
> 
> MaxMemFree   2560
> 
> but then I would expect some kind of "out of memory" errors in the logs?
> Another question: what is the good choice of MaxMemFree value? We want to
> leave 512MB-1GB or RAM on this machine for other tasks.
> 
> 
> 
> 
> 
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