Autoloading PHP Classes to Reduce CPU Usage

Posted by Mike Brittain on March 27, 2008
PHP / 3 Comments

At Heavy, we have been using a database ORM called Propel for some time now. I’m not a huge fan, to be honest, but it is what it is. One issue that we ran into is that Propel generates about five class files for every table that you model in PHP. With a few hundred tables, there are quite a few class files in our database code. We use Propel in an application layer behind our front-end web servers. Given that some of the controller scripts for the application layer might handle a variety of requests and deal with a large number of tables, you might imagine that a single script could end up requiring 50-200 class files at runtime.


Last fall, we took a close look at how to split up this load and only compile the truly necessary PHP class files when needed. We setup an autoload routine and saw dramatic results. We went from a typical 175% CPU usage on each 4 processor server down to around 50%. Much of the CPU seems to have been eaten up by constant recompiling of this huge number of class files. (Note: we also looked at using APC in the past to cache compiled PHP code, but ran into regular segfault issues on Apache when running it, which I believe are a fault of our own legacy configurations.)

Our autoloader looks something like this:

function __autoload ($class_name)
  if (!class_exists($class_name, false)) {
    $class_file_path = str_replace('_', '/', $class_name) . '.php';

There is an assumption, here, that you’ve got your PHP classes in order. One class per file, and using a PEAR-ish naming convention, where the class Input::Validator is stored in as Input/Validator.php in your include path.

If your class files are always located in one library directory, you can get some extra points by using an absolute path in your autoloader so that PHP doesn’t have to search your configured include path. For us, we’ve modeled our include path so that we can use custom classes, Zend Framework classes, and PEAR classes together. This ensures that our custom libraries override calls for similarly named classes in Zend or PEAR (which can be bad if you’re not careful!).

  1. Current working directory (.)
  2. Custom library path
  3. Zend Framework path
  4. PEAR path
  5. PHP standard path

Since we use this across our code base, we included the autoload function in a script that gets automatically loaded for every PHP script using auto_prepend_file. I can hear the groans already. The truth is that this works very well for our codebase. Your mileage may vary.

What does this gain us?

First of all, we improved the overall performance of our application, and I think pretty dramatically. Figures 1 and 2 (below) show Munin reports of our CPU usage on front-end servers and application servers. The drops in October and November coincide with our code promotions that included class autoloading.

Simplified development is another benefit. We never have to think about whether we’ve already included a class file in a script we’re writing, or whether it was conditionally included somewhere other than the top of the script file. If the class hasn’t been defined, the autoloader takes care of loading the file.

Finally, classes that fall out of use in our codebase automatically fall out of the runtime code. Unless we use a class, it’s class file is never included in the running script. When managing a lot of legacy code, possibly written by people who no longer work with you, this works great for culling out the old classes.

CPU Usage for Web Server
Figure 1. CPU usage for front-end web server

CPU Usage for Application Server
Figure 2. CPU usage for back-end application server

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