Apache is leaking semaphores when running mod_mono

Question

I'm running an ASP.NET MVC2 application under mod_mono with mono 2.8.1 and currently have to periodically clear out semaphore arrays that apache seems to be leaking.

I started with mono rpm's for 2.6.7 a while back but had had some issues both with leaking semaphore arrays (i.e. more and more accumulating in ipcs) and some incompatibility with ASP.NET MVC2, so I built 2.8 from source. The leak continued, so I just built 2.8.1 from source and the same thing is still happening. This is on an Amazon AMI (i guess it's centos under the hood). The symptoms are that semaphore arrays keep building up and if i don't manually remove them with ipcrm after a while requests to ASP.NET pages return no content with no errors in the logs. I've also reproduced the same issue in an centos 5.4 AMI.

Is anyone successfully running ASP.NET under apache/mod_mono and I'm just running into some edge case? Since I can't find any mentions of this happening to anyone else, I assume it's not general ASP.NET bug. Any ideas how i can troubleshoot this further?

Answer 1

Finally figured this out and while the solution exposes my own mistake of not following up other warning I was receiving, i figure this should be useful to anyone else runnning into this.

By default apache config has the below config order:

Include conf.d/*.conf
User apache
Group apache

I.e. all conf files (usually where vhosts are defined) are loaded before httpd user and group are set. This results in the below warning on restart:

[Mon Jan 24 00:12:50 2011] [crit] The unix daemon module not initialized yet.
Please make sure that your mod_mono module is loaded after the User/Group
directives have been parsed. Not initializing the dashboard.

While everything seems to work anyhow, this is the cause for the semaphore leak. If you move the Include after the User/Group, the warning goes away and mod_mono no longer leaks semaphores.

Answer 2

I've seen this with the shared memory used by cross-process handles.

My fix was to set MONO_DISABLE_SHM=1, however I'm not sure if this is your problem since the cross-process handle support is disabled starting with 2.8.

You could probably still try MONO_DISABLE_SHM to see if it makes a difference.