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From Jeff Trawick <traw...@gmail.com>
Subject Re: [users@httpd] MaxClients exceeded error message
Date Wed, 25 Jun 2014 14:38:31 GMT
On Wed, Jun 25, 2014 at 8:52 AM, Robin Becker <robin@reportlab.com> wrote:

> An apache 2.2 mpm prefork  site which uses an external mod_fastcgi socket
> to django was experiencing slowdowns during a busy period.
>
> The main symptom seemed to be a connection delay of up to 10 seconds.
>
> At the time I saw the machine was not heavily loaded in cpu or memory, but
> there were a large number of apache processes; stupidly I didn't think to
> count them.
>
> Later I looked for an error message related to MaxClients exceeded or
> similar, but could not see any.
>

It only appears once per generation, so it might have been days before you
noticed the symptom.  (Generation: each fresh start or restart of the
server is a generation.  This can be viewed from mod_status.)


> I have upped the MaxClients setting from 150 to 300 and in another busy
> period don't see the slowdown although monitoring the apache process count
> shows it can exceed the old limit.
>
> The django side of this is not especially fast and the single socket
> connection seems like it might be a choke point, but my understanding is
> that the django side of the fastcgi is also spawning children to handle the
> requests.
>
> What are the effects of MaxClient limit being exceeded?


delays in processing new connections

depending on load and MaxClients and ListenBacklogs, some new connections
might even get rejected



> Should I have seen an error message if it was exceeded?


once per generation

--/--

It can be useful to log the response time in microseconds (%D in log
format) in order to distinguish delays in the application (delay is
reflected in value for %D) from delays before httpd starts processing it
due to capacity tuning (delay not reflected in value for %D since it
happened before processing started)


> --
> Robin Becker
>
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