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From Jeff Beard <j...@cyberxape.com>
Subject Re: "AcceptMutex sysvsem" on high-volume Solaris servers
Date Fri, 12 Jul 2002 14:12:55 GMT
You should probably post this up on the httpd docs list. In fact, if 
you're really interested in having this information include, DIY then 
post a diff.

--Jeff

Mark Mentovai wrote:
> This information applies to Apache running on Solaris with AcceptMutex 
> set to sysvsem, and offers a method to improve Apache's performance.  
> It may apply to other systems as well, if you replace some of the 
> particulars (like "/etc/system") with values that make sense.
> 
> On Solaris, Apache makes available three methods for accept 
> serialization: fcntl, sysvsem, and pthread.  "AcceptMutex fcntl" is the 
> default and "safest" method in that is most likely to function as 
> expected, but is comparitively a poor performer.  Using System 5 
> semaphores by specifying "AcceptMutex sysvsem" offers superior 
> performance, but as discussed in the past, does not function on high-
> volume servers.  (See htdocs/manual/misc/perf-tuning.html for a 
> discussion of accept serialization.)
> 
> On an untuned Solaris system, configuring "AcceptMutex sysvsem" will 
> eventually cause Apache to exit, leaving the log peppered with:
> 
> accept_mutex_on: No space left on device
> [Mon Jul  8 12:23:24 2002] [alert] Child 2319 returned a Fatal error... 
> Apache is exiting!
> 
> This occurs once 30 child processes are present.  It causes the parent 
> to exit, leaving 30 children without any pool size regulation: you're 
> stuck with 30, unless the children exit (either cleanly because of a 
> nonzero "MaxRequestsPerChild" setting or for some other reason), in 
> which case you'll be stuck with fewer.  The parent process probably 
> shouldn't exit in this manner, but that's probably another discussion 
> for another forum.  Regardless, sysvsem locking will not work with more 
> than 30 processes without tuning.
> 
> The problem is that by default, Solaris (at least through Solaris 8, I 
> haven't yet tried 9) is configured to support a maximum of 30 semaphore 
> undo structures.  Each child calls semop in accept_mutex_on_system with 
> sem_flags & SEM_UNDO, and therefore accounts for one such structure.  
> When the 30th child calls semop, it fails with ENOSPC (see "man semop") 
> and the parent exits.  This is obviously insufficient.
> 
> By tuning the seminfo_semmnu parameter in the semsys driver, it is 
> possible to avoid this problem.  Adding the following to /etc/system 
> and rebooting will increase the number of undo structures from its 
> default of 30:
> 
> set semsys:seminfo_semmnu = 1024
> 
> It might be useful if this information was included in htdocs/manual/
> misc/perf-tuning.html or htdocs/manual/misc/performance.html.
> 
> On a related note, htdocs/manual/misc/perf-tuning.html indicates that 
> "AcceptMutex pthread" only gives appropriate behavior under some 
> Solaris configurations.  Which configurations are these?  Is this 
> information even current?  The document refers somewhat incorrectly to 
> the HAVE_*_SERIALIZED_ACCEPT macros in a way that makes it unclear 
> whether or not it's been updated aside from a quick :s///g.  Using the 
> pthread interface for accept serialization would avoid some of the 
> drawbacks of semaphores, and could speed up the locking.
> 
> Mark
> 
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