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From Jeff Trawick <>
Subject Re: accept mutex failure causes fork bomb
Date Tue, 15 Sep 2009 12:07:43 GMT
On Mon, Sep 14, 2009 at 4:27 PM, Greg Ames <> wrote:

> I'm trying to debug a problem where apparently the accept mutex went bad on
> a z/OS system running the worker MPM.  I'm guessing that some memory that we
> use for the semaphore got clobbered but don't have proof yet.  The error log
> looks like:
> [Mon Sep 07 08:01:59 2009] [emerg] (121)EDC5121I Invalid argument.:
> apr_proc_mutex_unlock failed. Attempting to shutdown process gracefully.

Could it be some system limit exceeded, like max waiters on a mutex?  (IIRC
some OSs require tuning of SysV semaphores.  (Older Solaris comes to mind.))

> [Mon Sep 07 08:02:01 2009] [emerg] (121)EDC5121I Invalid argument.:
> apr_proc_mutex_lock failed. Attempting to shutdown process gracefully.
> [Mon Sep 07 08:02:02 2009] [emerg] (121)EDC5121I Invalid argument.:
> apr_proc_mutex_lock failed. Attempting to shutdown process gracefully.
> [Mon Sep 07 08:02:02 2009] [emerg] (121)EDC5121I Invalid argument.:
> apr_proc_mutex_lock failed. Attempting to shutdown process gracefully.
> [...]
> The rest of the error log is filled with lock failures.  Looking at the
> time stamps, you can see that perform_idle_server_maintenance went into
> exponential expansion, maxing out at about 24 lock failures per second.
> Unfortunately the fork()s were faster than z/OS could terminate the
> processes that had detected the mutex problem, so after forking 978 httpd
> children, the system ran out of real memory and had to be IPLed.
> One of my colleagues asked why ServerLimit 64 didn't stop the fork bomb.
> Good question.  The reason is that the error path calls signal_threads()
> which causes the child to exit gracefully.  The listener thread sets
> ps->quiescing on the way out, which allows the "squatting" logic in
> perform_idle_server_maintenance to take over the scoreboard slot before the
> previous process has completely exited, bypassing the ServerLimit throttle.
> This raises several ideas for improvement:
> * Should we do clean_child_exit(APEXIT_CHILDSICK or CHILDFATAL) for this
> error?  We have a previous fix to detect accept mutex failures during
> restarts and tone down the error messages.  I don't recall seeing any false
> error messages after that fix.  We could also use requests_this_child to
> detect if this process has ever successfully served a request, and only do
> the clean_child_exit if it hasn't.

So nasty failures prior to successfully accepting a connection bypass
squatting?  Good.

CHILDSICK or CHILDFATAL in that case?  In this example it probably wasn't
going to get any better.  However, I think it is reasonably likely that
child process n+1 encounters some sort of resource limit, so CHILDSICK seems

> * Should we yank the squatting logic?  I think it is doing us more harm
> than good.  IIRC it was put in to make the server respond faster when the
> workload is spikey.  A more robust solution may be to set Min and
> MaxSpareThreads farther apart and allow ServerLimit to be enforced
> unconditionally.  disclaimer: I created ps->quiescing, so I was an
> accomplice.

My understanding is that squatting is required to deal with long-running
requests that keep a child trying to exit from actually exiting, thus tying
up a scoreboard process indefinitely.  (We've had the assumption that we
didn't want to drastically overallocate scoreboard to accommodate a bunch of
children with only a few threads handling requests.)

Is it reasonable to have up to MaxClients worth of squatting like we have
now?  (I think that is what we allow.)  No, I don't think so.

Should we axe squatting, respect ServerLimit, and thus make the admin raise
ServerLimit to accommodate exiting processes which are handling long-running
requests (and waste a bit of shared memory at the same time)?  Maybe ;)
 That change seems a bit drastic, but I'm probably just scared of another
long period of time before I halfway understand how it behaves in the real

> * Does it make sense to fork more than MaxSpareThreads worth of child
> processes at a time?  MaxSpareThreads was 75 in this case, but we tried to
> fork at least 600 threads (same as MaxClients) worth of child processes in
> one pass of perform_idle_server_maintenance.

That's a good idea.

> This applies to worker and event; some of it may also apply to prefork.
> I'd appreciate thoughts and suggestions before committing anything.
> Thanks,
> Greg

Born in Roswell... married an alien...

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