My server (ubuntu 8.04) LAMP running drupal 6, when there is high traffic, it stops serving pages. A restart of apache2 will not work, so I have to restart the service.
I found this message in the apache2 error.log
[notice] caught SIGWINCH, shutting down gracefully
Also I notice that the process id of the apache2, was around 12000 when apache stops responding.
Update
The caught SIGWINCH, as you point out, is a notice of the apache service restarting. I played around with the max clients and keep alive. I set the MaxClients too low so I got a "server reached MaxClients setting, consider raising the MaxClients setting" error, but fixed that.
Regarding the PID, my installation has prefork module installed, so after MaxRequestPerChild is reached, it recycles the child process. That is why the PID is rising periodically. Still haven't figured out why after a while apache stops responding.
In the mean time will try to rise the MaxRequestPerChild so the PID dosen't reach PID max that fast. Current limit is 32768 (which is standard for new debian and ubuntu installations).
Here is more from the log:
[Fri Apr 24 01:48:56 2009] [notice] caught SIGWINCH, shutting down gracefully
[Fri Apr 24 01:50:07 2009] [notice] Apache/2.2.8 (Ubuntu) PHP/5.2.4-2ubuntu5.3 with Suhosin-Patch configured -- resuming normal operations
[Fri Apr 24 17:13:35 2009] [error] [client 195.70.62.131] client sent HTTP/1.1 request without hostname (see RFC2616 section 14.23): /w00tw00t.at.ISC.SANS.DFind:)
[Fri Apr 24 17:36:00 2009] [error] [client 212.188.33.4] client sent HTTP/1.1 request without hostname (see RFC2616 section 14.23): /w00tw00t.at.ISC.SANS.DFind:)
[Sat Apr 25 20:05:07 2009] [error] [client 84.243.222.12] Invalid URI in request GET HTTP/1.1 HTTP/1.1
[Sat Apr 25 20:05:12 2009] [error] [client 84.243.222.12] script not found or unable to stat: /usr/lib/cgi-bin/twiki
[Sat Apr 25 20:05:12 2009] [error] [client 84.243.222.12] script not found or unable to stat: /usr/lib/cgi-bin/wiki
[Sat Apr 25 20:05:12 2009] [error] [client 84.243.222.12] script not found or unable to stat: /usr/lib/cgi-bin/wikis
[Sat Apr 25 21:46:46 2009] [error] [client 211.68.23.167] Invalid URI in request GET HTTP/1.1 HTTP/1.1
[Sun Apr 26 06:13:47 2009] [error] [client 86.39.154.89] client sent HTTP/1.1 request without hostname (see RFC2616 section 14.23): /w00tw00t.at.ISC.SANS.DFind:)
[Sun Apr 26 06:53:07 2009] [notice] Graceful restart requested, doing restart
Well, SIGWINCH is used in some situations when starting and stopping Apache on Ubuntu. If I do sudo apache2ctl graceful-stop on my Ubuntu 8.10 server, I get this in my log:
[Fri Apr 24 22:41:15 2009] [notice] caught SIGWINCH, shutting down gracefully
As I remember it, SIGWINCH is also used by some services that need to restart Apache when rotating logs, nightly jobs, etc.
That doesn't explain the problem you're currently having, but I think it might be something else running on your server that's restarting Apache – or it might not be related to your problem at all.
Try posting more of the logfile if you can, it'll be a lot easier to analyse :)