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Date: Thu, 24 Mar 2016 10:08:44 +0100
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Subject: Re: apr_global_mutex_create vs apr_proc_mutex_create
From: Luca Toscano <toscano.luca@gmail.com>
To: modules-dev@httpd.apache.org, dev@httpd.apache.org
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+httpd-dev@

Hello!

2016-03-23 21:49 GMT+01:00 Ali Shah <caffein@gmail.com>:

> Hi Module maintainers,
>
> I'm writing a simple apache module and I'd like to aggregate some
> statistics.
> I saw an example module that does this (
> https://wiki.apache.org/httpd/ModuleLife) using shared memory.
>
> My question for those on this list following:
>
> What is the main difference between using mutexes created
> via apr_global_mutex_create and apr_proc_mutex_create in terms of
> performance and safety of the resource that being protected?
>
> The docs seem to discourage usage of apr_global_mutex_create:
> "There is considerable overhead in using this API if only cross-process or
> cross-thread mutual exclusion is required."
>
> Is the overhead purely in code setup (since global mutex should be
> initialized in post_config), or is this describing some performance
> penalties as well?
>

>From what I can see in [1] a global lock uses both apr_proc_mutex_create
and apr_thread_mutex_create, so this is why the documentation suggests the
use of a more specialized one if only cross-process or cross-thread sync is
needed.

Not a great explanation I know, somebody else with more experience will
give you a better answer on this list :)

Luca


[1]:
https://github.com/apache/apr/blob/7951861fb029fd0fb60e5ec8da73015d03afa28d/locks/unix/global_mutex.c#L46

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