[mod_python] mod_python or apache scalability?

Jim Gallacher jpg at jgassociates.ca
Mon Oct 1 07:54:49 EDT 2007

Are you using Sessions, specifically DbmSession?


Alec Matusis wrote:
> I am sorry in advance if this turns out to be an apache-related issue, but
> when I posted this on apache list, it has been suggested that it might be an
> application issue, so I am reposting it here.
> We are running a busy mod_python/3.1.4 Python/2.4.1 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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