Linux – How to investigate a memory leak with Apache and PHP

apache-2.2drupallinuxmemoryPHP

We're running a heavy Drupal website that performs financial modeling. We seem to be running into some sort of memory leak given the fact that overtime the memory used by apache grows while the number of apache processes remains stable:

enter image description here

enter image description here

We know the memory problem is coming from apache/PHP because whenever we issue a /etc/init.d/httpd reload the memory usage drops (see above screenshot and below CLI outputs):

Before httpd reload

$ free
             total       used       free     shared    buffers     cached
Mem:      49447692   45926468    3521224          0     191100   22609728
-/+ buffers/cache:   23125640   26322052
Swap:      2097144     536552    1560592

After httpd reload

$ free
             total       used       free     shared    buffers     cached
Mem:      49447692   28905752   20541940          0     191360   22598428
-/+ buffers/cache:    6115964   43331728
Swap:      2097144     536552    1560592

Each apache thread is assigned a PHP memory_limit of 512MB which explains the high memory usage depiste the low volume of requests, and a max_execution_time of 120 sec which should terminate threads which execution is taking longer, and should therefore prevent the constant growth in memory usage we're seeing.

Q: How could we investigate what is causing this memory leak?

Ideally I'm looking for troubleshooting steps I can perform on the system without having to bother the dev team.

Additional info:

OS: RHEL 5.6
PHP: 5.3
Drupal: 6.x
MySQL: 5.6

FYI we're aware of the swapping issue which we're investigating separately and has nothing to do with the memory leak which we've observed before the swapping started to occur.

Best Answer

We know the memory problem is coming from apache/PHP because whenever we issue a /etc/init.d/httpd reload the memory usage drops

No - that just means it's related to the web traffic. You've gone on to mention that you're running mysql on the box - presumably managing data for the webserver - it could just as easily be the culprit here. As could other services your webstack uses which you've not mentioned.

Each apache thread is assigned a PHP memory_limit of 512MB which explains

No it doesn't. You're reporting an average of 7 and a max of 25 busy servers - yet your memory graph shows a delta of around 25Gb.

Really you should start again with basic HTTP tuning - you seem to be running a constant 256 httpds, yet your peak usage is 25 - this is just plain dumb.

and a max_execution_time of 120 sec which should terminate threads which execution is taking longer

No - only if the thread of execution is within the PHP interpreter - not if PHP is blocked.

that performs financial modeling

(sigh)

It would have been helpful if you'd provided details of how you have configured Apache, threaded or prefork, what version, how PHP is invoked (module, cgi, fastcgi), whether you are using persistent connections, whether you use stored procedures.

I'd suggest you start by moving mysql onto a seperate machine and stop using persistent connections (if you're currently using them). Set the memory limit much lower and override this on a per-script basis. Make sure you've got the circular reference garbage collector installed and configured.

Related Topic