In a reference counted environment there are times you need to release an object after it has passed out of the reach of the method or object that created it. To solve this Cocoa has the concept of an autorelease pool. A developer can set an object to be auto released, which means it is added to one of these pools which is periodically drained.

If you are creating an AppKit application, then you have an autorelease pool set up for you so you don't need to worry about it most of the time. If you set up a Foundation tool then you'll find that the Xcode template puts code to initialise and release an autorelease pool at the beginning and end of your main function.

However, a problem arises when you're creating a lot of objects at once. The obvious solution is to initialise and release objects by hand in this case, but sometimes it isn't possible. A lot of objects returned by Cocoa methods are autoreleased (by convention any object returned by a class method (other than +new or +alloc) should be autoreleased).

I had one of these situations in Code Collector Pro. Some users were having problems when pasting in the jQuery code, so I decided to investigate. I found that CCP's memory usage was jumping up by about 800MB when performing the regexes to find what code to colour. Obviously that's bad.

The problem is, there were a huge number of autoreleased objects being created, but the autorelease pool only releases all the objects it contains at the end of each cycle of the run loop. All these objects are being created on once cycle.

The solution? Make another pool just for these objects. Autorelease pools are stored in a stack (one per thread). If you initialise a new one all objects autoreleased until you release the pool will go into it. The code that was creating objects was running in a loop so it made sense to release the objects after every iteration of the loop. So I changed my code from:

for (id item in array) {
    //Perform regexes and colour
}

for (id item in array) {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    //Perform regexes and colour
    [pool drain];
}

This reduced my max memory usage from the 841.07MB show above to 22.33MB. You can also see below how there are many peaks and troughs in the memory taken up by objects now. These represent each iteration of the loop, where a lot of objects are created and then released.

Good use of NSAutoreleasePools in areas where you are creating lots of objects can reduce your memory footprint a huge amount, especially in loops.

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