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Posted at 2011-09-16 14:00 | RSS feed (Full text feed) | Blog Index
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Friday Q&A 2011-09-16: Let's Build Reference Counting
by Mike Ash  

Last time, I discussed how to build NSAutoreleasePool and how it works internally. Today, I'm going to carry that theme forward by building an implementation of Cocoa reference counting with retain and release, a topic suggested by David Dunham.

Background
Everyone reading this probably already has a basic grasp of what reference counting is, but it's good to discuss it in detail to make sure the relevant concepts are clear.

Every Objective-C object has an associated reference count. In Cocoa terminology this is referred to as a "retain count", but I think that just confuses matters so I'll use "reference count". A newly created object starts out with a reference count of 1. A retain message increments it, and a release message decrements it. If release decrements the reference count to zero, the object is destroyed by sending it the dealloc message.

Because reference counting is implemented with plain Objective-C messages, it can be overridden by a particular class. As long as that override follows the above semantics, everything still works.

For what are now largely historical reasons, the default reference counting implementation in NSObject does not store the reference count in the object itself. Instead, the reference count is stored in an external table. This can save some memory at the cost of speed. I will replicate this table-based approach in my reimplementation of reference counting.

CoreFoundation objects and many other Cocoa objects override retain and release in order to store the reference count in the object itself instead, which increases performance a bit.

Lion and ARC
Lion and ARC both saw some substantial changes in the Cocoa memory management system, but the basics discussed above still hold true. Lion reimplemented the table-based reference count system to make it significantly faster, but the fundamental idea is still the same.

ARC still ends up doing retain and release behind the scenes. For performance reasons, it actually calls runtime functions to retain and release which are able to bypass Objective-C message sends in some cases. However, it still respects retain and release overrides and conceptually ARC is still just sending retain and release messages to the objects that it manages.

Source Code
As usual, the code for today is available on GitHub here: https://github.com/mikeash/refcounting

Design
Before I get into the code itself, let's talk about the specifics of this design. There are two methods which need to be implemented. To keep mine separate from Cocoa's, I'll call mine ma_retain and ma_release.

The reference count table will be implemented using a CFMutableDictionary mapping object pointers to integers. This provides a reasonably fast and direct approach.

To make life simpler, the reference count of 1 will be indicated by not having an entry in the table at all. In other words, any pointer's reference count is 1 by default. This means that new objects get the correct reference count of 1 automatically. This is how Cocoa's implementation works as well.

Thread safety is critical for any reference counting implementation. A class typically has no control over which threads it gets retained and released on, and even an otherwise thread-unsafe class needs to be able to withstand being retained and released on separate threads simultaneously, since it's extremely difficult to track and control where retains and releases happen. To make the CFMutableDictionary safe, I protect it with an OSSpinLock. Any lock will do, but spinlocks are well suited to this case, since they are cheap as long as the critical region is fast.

There are two primitive operations on the table: GetRefcount and SetRefcount. These take care of the grunt work of manipulating the table, of treating the lack of an entry as 1, removing objects from the table when their reference count drops below 2, and such. They do not lock the spinlock, and that is left up to the caller. This is necessary because both ma_retain and ma_release need to perform two operations on the table atomically, so they need to lock around both.

Building on top of these two primitives are two slightly higher level functions: IncrementRefcount and DecrementRefcount. IncrementRefcount takes a pointer and simply increases that pointer's reference count in the table. DecrementRefcount takes a pointer and decrement its reference count. It also returns the new count, which is important for implementing deallocation.

Finally, there are the methods, which are thin wrappers around these higher level functions. ma_retain just calls IncrementRefcount. ma_release calls DecrementRefcount, and if the return value is 0, calls [self dealloc].

Code
Tired of words? Let's do some code.

Two global variables are used for the reference count table. One is the dictionary for the table itself, and the other is the spinlock:

    static CFMutableDictionaryRef gRefcountDict;
    static OSSpinLock gRefcountDictLock;

The GetRefcount function retrieves the current reference count for the given pointer. The gRefcountDict variable is initialized lazily, on demand, so the first thing this function does is check to see if the dictionary has been initialized yet. If it hasn't, then it knows that no object has a reference count yet, so the reference count for this object must be 1:

    static uintptr_t GetRefcount(void *key)
    {
        if(!gRefcountDict)
            return 1;

If the dictionary does exist, it grabs the value out of it. If the value isn't present, it returns 1 again. Otherwise it returns whatever value is in the dictionary:

        const void *value;
        if(!CFDictionaryGetValueIfPresent(gRefcountDict, key, &value))
            return 1;

        return (uintptr_t)value;
    }

The SetRefcount function sets the reference count for the given pointer to the given value. If the value is 1 or 0, it simply removes the object from the table:

    static void SetRefcount(void *key, uintptr_t count)
    {
        if(count <= 1)
        {
            if(gRefcountDict)
                CFDictionaryRemoveValue(gRefcountDict, key);
        }

If the table hasn't been created yet then there's nothing to do, since there can't be any entry to remove.

If the value to set is greater than 1, it first lazily initializes the table, then sets the object's value in it:

        else
        {
            if(!gRefcountDict)
                gRefcountDict = CFDictionaryCreateMutable(NULL, 0, NULL, NULL);
            CFDictionarySetValue(gRefcountDict, key, (void *)count);
        }
    }

The IncrementRefcount function takes a pointer and increments its value in the table. To do this, it first acquires the spinlock. With the lock held, it fetches the current value, adds one, and then sets that new value. Finally, it releases the spinlock:

    static void IncrementRefcount(void *key)
    {
        OSSpinLockLock(&gRefcountDictLock);
        uintptr_t count = GetRefcount(key);
        SetRefcount(key, count + 1);
        OSSpinLockUnlock(&gRefcountDictLock);
    }

The DecrementRefcount function is much the same. The only real difference is that it needs to return the new count. To make this work, it saves the new count in a temporary variable, then returns it at the end of the function:

    static uintptr_t DecrementRefcount(void *key)
    {
        OSSpinLockLock(&gRefcountDictLock);
        uintptr_t count = GetRefcount(key);
        uintptr_t newCount = count - 1;
        SetRefcount(key, newCount);
        OSSpinLockUnlock(&gRefcountDictLock);

        return newCount;
    }

Next up are the methods which wrap these functions. These are simple and mostly self explanatory:

    @implementation NSObject (MARefcounting)

    - (id)ma_retain
    {
        IncrementRefcount(self);
        return self;
    }

    - (void)ma_release
    {
        uintptr_t newCount = DecrementRefcount(self);
        if(newCount == 0)
            [self dealloc];
    }

    @end

Note that ma_retain returns self to stick with the Cocoa tradition of allowing statements like this:

    object = [otherObject retain];

It's not an intrinsic requirement of the reference counting system.

That's all there is to the custom reference counting system!

To test it, I wrote some quick code that generates a bunch of objects, retains and releases them in a stress test, and makes sure that they get deallocated. I won't reproduce it here, but you can view that code on GitHub here: https://github.com/mikeash/refcounting/blob/master/refcounting.m#L80

Conclusion
As with autorelease pools, with this simple implementation we can pull back the curtain from Cocoa memory management and see that there is no wizard and no mystery behind the scenes. It's just a simple counting system using standard Objective-C messaging. Cocoa's system has plenty of tricks to make it fast, but fundamentally it's the same as this quick and simple version. Again, we can see the answers to simple questions like "What happens if I retain an object twice?" or "Can I retain an object on one thread and release it on another thread?" If you retain an object twice, you need to release it twice, since it's just a simple increment and decrement. Retain and release don't care what thread you're on, so as long as they balance and synchronize correctly, everything works fine.

That wraps up today's expedition. Come back in two weeks for part three of this journey through the Cocoa memory management system with a discussion of ARC, how it works, and how to use it. As always, Friday Q&A is driven by reader suggestions, so if you have a topic that you would like to see covered, send it in!

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Comments:

Chris Lattner at 2011-09-16 17:25:28:
Thanks for the very educational article Mike!

I'd just like to point out a couple of very important things: first, this implementation is not compatible with "zeroing weak" pointers in ARC: they will explode at runtime or malfunction if you try to use them with classes like this. Second, this is not actually faster than NSObject's implementation, and defeats the runtime optimizations that happen in objc_retain and friends.

In other words, I think it's great that you peeled off the covers to show how this stuff conceptually works, but it would be a bad idea for someone to actually go implement their own retain/release in a real app.

-Chris

mikeash at 2011-09-17 00:30:41:
That's an excellent point. You'd never want to build an implementation like this for real, unless of course you were reimplementing reference counting from scratch for some other environment. There have been performance advantages for redoing reference counting to use inline storage, but it sounds like that's now largely moot on 10.7, and in any case still breaks the weak reference stuff.

ptx at 2011-09-17 16:03:30:
The DecrementRefcount function unlocks the spinlock before the reference count is returned, couldn't this mean that if ma_retain is called from another thread just as me_release have read the reference count that the object is deallocated by mistake?

mikeash at 2011-09-17 16:42:53:
It's an error to retain, or indeed manipulate an object in any way, from one thread while another thread is in the process of releasing the last retain on it. You have to do your own locking to synchronize things in that case, or better yet have both threads do their own retains.

Dominic Hopton at 2011-09-27 17:51:44:
n00b question:
Why would you not hold the count on the object? Wouldn't that allow you to use CPU optimizations such as atomic increment/decrement operations, rather than the giant lock you've got here, as well as not needing the table look ups. (isn't that single global lock a bottle neck across large [de]allocations?)

mikeash at 2011-09-28 16:20:08:
If I were building my own implementation for practical purposes, I would definitely use an inline reference count as you describe. However, my intent was to replicate what Apple does, and Apple uses a big global table.

The question then becomes, why does Apple use that? I can only speculate, but I think it probably made a great deal more sense back when this stuff was first built. Multithreaded performance was completely unimportant at the time, but memory was at a premium. An inline reference count means an additional four bytes in every object. A global table with "no entry" being interpreted as 1 could represent a significant savings if retain counts >= 2 are rare, as they probably are in many programs.

gparker at 2011-09-28 19:10:37:
A common bug is to call -dealloc a second time if the implementation of -dealloc retains and releases self. Your implementation does not have this bug. Exercise: why not?


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