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From "Alec Matusis" <matu...@yahoo.com>
Subject RE: [users@httpd] requests time-out under load, no warnings in logs
Date Mon, 01 Oct 2007 09:00:25 GMT
Hi Christian, 

> How does your apache look memory-wise in top/htop?

When Apache is "slow", it has about 200 processes with 11-15MB RSS each, soat least 1GB of
physical RAM should be unused.
The load average drops from normal 16.0 (this is a 4 CPU machine) to only 6.0 when apache
becomes "slow".


It would be interesting to see an example of a reasonable choice for MaxMemFree, after 2 days
of searching in Google I cannot find any guidelines for setting it. 



> -----Original Message-----
> From: Christian Folini [mailto:christian.folini@post.ch]
> Sent: Sunday, September 30, 2007 11:55 PM
> To: users@httpd.apache.org
> 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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> 
> ---------------------------------------------------------------------
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