Circular References

Circular-references has been a long outstanding issue with PHP. They are caused by the fact that PHP uses a reference counted memory allocation mechanism for its internal variables. This causes problems for longer running scripts (such as an Application Server or the eZ Components test-suite) as the memory is not freed until the end of the request. But not everybody is aware on what exactly the problem is, so here is a small introduction to circular references in PHP.

In PHP a refcount value is kept for every variable container (zval). Those containers are pointed to from a function's symbol table that contains the names of all the variables in the function's scope. Every variable, array element or object property that points to a zval will increase its refcount by one. The refcount of a zval container is decreased by one whenever call unset() on a variable name that points to it, or when a variable goes away because the function in which it was used ends. For a more thorough explanation about references, please see the article on this that I wrote for php|architect some time ago.

The problems with circular references all start by creating an array or an object:

<?php
$tree = array( 'one' );
?>

This creates the following structure in memory:

Now if we proceed to add a new element to the array, that points back to the array with:

<?php
$tree[] = $tree;
?>

We create a circular reference like this:

As you can see there are two variable names pointing to the array itself. Once through the $tree variable, and once through the 2nd element of the array. Because there are two variable names pointing to the container, its refcount is 2.

Now, the next step that actually creates the problem if we unset() the $tree variable. As I mentioned before an unset() on a variable name will decrease the refcount of the variable container the variable points to, in this case the array value. The structure in memory now looks like this:

Note that there is no variable pointing to the array container anymore. Because of this PHP has no way of freeing this data anymore, and the memory leak is born. PHP however does remove all allocated variables at the end of each request, so it's not a hard-memory leak, but it's still annoying enough for long running scripts and daemons written in PHP.

Luckily there are a few solutions to this problem. The first one is to use a new garbage collection algorithm, another is to augment the reference counting system with some cyclic tracing . The latter solution is something that is currently undertaken by David Wang, one of the Google Summer of Code students. He mentioned he is making good progress and I can hardly wait until I can play with it :)