How to Manage Memory in PHP | 3 | WebReference

How to Manage Memory in PHP | 3

How to Manage Memory in PHP

Reference Counting

Careful memory allocation and freeing is vital to the long term performance of a multirequest process like PHP, but it's only half the picture. In order for a server that handles thousands of hits per second to function efficiently, each request needs to use as little memory as possible and perform the bare minimum amount of unnecessary data copying. Consider the following PHP code snippet:

After the first call, a single variable has been created, and a 12 byte block of memory has been assigned to it holding the string ‘Hello World' along with a trailing NULL. Now look at the next two lines: $b is set to the same value as $a, and then $a is unset (freed). If PHP treated every variable assignment as a reason to copy variable contents, an extra 12 bytes would need to be copied for the duplicated string and additional processor load would be consumed during the data copy. This action starts to look ridiculous when the third line has come along and the original variable is unset making the duplication of data completely unnecessary. Now take that one further and imagine what could happen when the contents of a 10MB file are loaded into two variables. That could take up 20MB where 10 would have been sufficient. Would the engine waste so much time and memory on such a useless endeavor?

You know PHP is smarter than that.

Remember that variable names and their values are actually two different concepts within the engine. The value itself is a nameless zval* holding, in this case, a string value. It was assigned to the variable $a using zend _hash_add(). What if two variable names could point to the same value?

At this point you could actually inspect either $a or $b and see that they both contain the string "Hello World". Unfortunately, you then come to the third line: unset($a);. In this situation, unset() doesn't know that the data pointed to by the $a variable is also in use by another one so it just frees the memory blindly. Any subsequent accesses to $b will be looking at already freed memory space and cause the engine to crash. Hint:You don't want to crash the engine.

This is solved by the third of a zval's four members: refcount. When a variable is first created and set, its refcount is initialized to 1 because it's assumed to only be in use by the variable it is being created for. When your code snippet gets around to assigning helloval to $b, it needs to increase that refcount to 2 because the value is now "referenced" by two variables:

Now when unset() deletes the $a copy of the variable, it can see from the refcount parameter that someone else is interested in that data and it should actually just decrement the refcount and otherwise leave it alone.

Copy on Write

Saving memory through refcounting is a great idea, but what happens when you only want to change one of those variables? Consider this code snippet:

Looking at the logic flow you would of course expect $a to still equal 1, and $b to now be 6. At this point you also know that Zend is doing its best to save memory by having $a and $b refer to the same zval after the second line, so what happens when the third line is reached and $b must be changed?

The answer is that Zend looks at refcount, sees that it's greater than one and separates it. Separation in the Zend engine is the process of destroying a reference pair and is the opposite of the process you just saw:

Now that the engine has a zval* that it knows is only owned by the $b variable, it can convert it to a long and increment it by 5 according to the script's request.


Created: March 27, 2003
Revised: June 19, 2006