WP Memcached Object Cache Leaks Memory?

While working on Pressbooks, a multi-site WordPress based web application, I noticed that some of our customers were getting blank pages in the admin section. Specifically, customers with a lot of Sites (or Books as they are known in Pressbooks).

Checking the error logs I saw that these customers were running out of memory.

PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 292913 bytes) in /path/to/object-cache.php on line 212.

First, to temporarily stop the out-of-memory problem so I could profile I added the following to wp-config.php:

define( 'WP_MAX_MEMORY_LIMIT', '512M' );

Next, using Blackfire.IO I was able to determine the following:

Before
Before (285 MB)

That is, when a customer was looking at their dashboard, PHP was consuming 285MB of memory. Most of it the Memcached Object Cache plugin.

That’s weird. I’m using the latest version of the plugin, the plugin is developed by core developers, and no one has reported this before? Or so I thought! Browsing the plugin SVN I see the following change committed to trunk:

Changeset 626248

There’s a few more fixes in there as well. After installing the TRUNK version of this plugin Blackfire.IO displayed:

After
After (12.2MB)

That’s a 273MB improvement!

It took me days to figure out this problem. It would have saved me a lot of time had I seen the new code first.

Bonus info:

  • The code in TRUNK has at least 2 bugs. (…just load the file in PHPStorm and the errors will be underlined in red)
  • Redis Object Cache gives better results.

For now, this is good enough.