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From Greg Ames <ames_g...@yahoo.com>
Subject Re: one word syncronize once more
Date Thu, 26 Jul 2007 15:33:57 GMT
the disease:  on an SMP system, multiple CPUs could try to update requests_this_child simultaneously,
and only one of the updates will take effect.  this is a rare timing situation.  the SMP issue
is not important because the variable will still be updated and continue to do its job.

requests_this_child is the variable used to implement MaxRequestsPerChild.  this is used to
periodically quiesce worker processes which are replaced by new workers and keep the server
going if there are slow memory or resource leaks due to bugs.  the hope is a worker process
that is using more and more memory or file descriptors over time will be shut down before
the situation gets serious.  it counts the number of connections served by a worker process,
contrary to its name.  

the admin picks a big number out of the air, let's say 10000, and sets MaxRequestsPerChild
to that value.  if he observes that that workers are now being shut down before the leak gets
too bad and there isn't a lot of unneccessary process forking/exiting, great!  he's done until
he has time to address the software bug which is the root cause of the problem.  if the workers
are still getting too big before quiescing, he lowers MRPC.  if there is no more worker growth
problem but the CPU is high and he can see workers getting forked very rapidly, he raises
MRPC.  so if internally the httpd actually processes 10002 connections instead of 10000 before
quescing some worker process, it isn't important enough to justify slowing down the mainline
path.

if you have a hung server, I would do some more diagnosis and not get sidetracked by MRPC.
 have you looked for unusual messages in the error log, especially messages that mention MaxClients?
 have you gotten samples of the server-status page with ExtendedStatus on?  do you see a pattern
with certain URLs that hang and others that do not?  are small local static files always served
quickly?

Greg
----- Original Message ----
From: Darryl Miles <darryl-mailinglists@netbauds.net>
To: dev@httpd.apache.org
Sent: Friday, July 20, 2007 1:46:53 PM
Subject: Re: one word syncronize once more

Greg Ames wrote:
> please see rev. 558039.  requests_this_child does not need to be 100% accurate.  the
cure below is worse than the disease.
> 
> Greg
> 
> -        requests_this_child--; /* FIXME: should be synchronized - aaron */
> +        apr_atomic_dec32(&requests_this_child); /* much slower than important */


Maybe someone could reconfirm to the list what exactly the disease is ?









       
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