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20070422 Sunday April 22, 2007

Linux Virtual Memory versus Apache

I ran into a very peculiar case of an Apache 2.0.x installation with the worker MPM completely failing to spawn it's configured thread pool. The hardware and kernel versions weren't significantly different from other systems running Apache with the same configuration. Here are the worker MPM params in use:

ServerLimit         40
StartServers        20
MaxClients        2000
MinSpareThreads     50
MaxSpareThreads   2000
ThreadsPerChild     50
MaxRequestsPerChild  0
But on this installation, same version of Apache and RedHat Enterprise Linux 4 like rest, every time httpd started it would cap the number threads spawned and leave these remarks in the error log:
[Fri Apr 20 22:54:24 2007] [alert] (12)Cannot allocate memory: apr_thread_create: unable to create worker thread 

It turns out that a virtual memory parameter had been adjusted, vm.overcommit_memory had been set to 2 instead of 0. Here's the explanation of the parameters I found:

overcommit_memory is a value which sets the general kernel policy toward granting memory allocations. If the value is 0, then the kernel checks to determine if there is enough memory free to grant a memory request to a malloc call from an application. If there is enough memory, then the request is granted. Otherwise, it is denied and an error code is returned to the application. If the setting in this file is 1, the kernel allows all memory allocations, regardless of the current memory allocation state. If the value is set to 2, then the kernel grants allocations above the amount of physical RAM and swap in the system as defined by the overcommit_ratio value. Enabling this feature can be somewhat helpful in environments which allocate large amounts of memory expecting worst case scenarios but do not use it all.
From Understanding Virtual Memory
The vm.overcommit_ratio value is set to 50 on all of our systems but rather than fiddling with that, setting vm.overcommit_memory to 0 had the intended effect; Apache started right up and readily stood-up to load testing.

So, if you're seeing these kind of evil messages in your Apache error log, use sysctl and check out the vm parameters. I haven't dug further into why the worker MPM was conflicting with this memory allocation config; next time I run into Aaron, I'm sure he'll have an explanation in his back pocket.


( Apr 22 2007, 08:19:57 PM PDT ) Permalink


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