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Alec Matusis
matusis at matusis.com
Mon Oct 1 05:12:26 EDT 2007
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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