Garbage collection in Flash Player 8

While this was not a feature I worked on I still want to point it out. Flash Player 8 contains a new garbage collection (GC) system for ActionScript. Due to the nature of the beast this is somehow a feature which does not get too much Ahhs and Ohhs. It is one of those important under the hood things you simply never think about or should have to think about. But it is one of the most challenging and complicated feature an engineer can work on, it took a whole release. It is simply amazing to watch an engineer look at some raw dump of memory and tell me what type of object it contains. I am sure that when he does day to day life mental arithmetic he uses hexadecimal instead of decimal. :-)

Flash Player 7 and lower used reference counting to keep track of ActionScript objects. That means that each object created in ActionScript had a tracking counter attached to it. If that counter became zero the object would be released and memory was freed. Every object means not only objects like MovieClips, Arrays etc, but also simple types like Numbers, Strings and Booleans. This is a very simple system and worked well in the past. With the advent of large applications done in Flash this model has extreme drawbacks: High memory usage since even simple types need a reference count, low performance since incrementing and decrementing the reference count takes time and the potential for circular references with the result that memory might never be freed (although weak references somewhat reduced this issue).

Flash Player 8 features a mark and sweep GC, very much like most of the implementations of the Java run time. While I could try explaining all the details (probably with little success due to my limited involvement), there is an excellent Wikipedia article on this subject if you are interested. One of the drawbacks of the mark and sweep model is that the collection of objects is not immediate but deferred until a later time. This means that it could block the player for an undetermined amount of time if there is a lot to collect. Early versions of the Java run time had this problem if you remember, sometimes everything would freeze up for many seconds. To get around this problem Flash Player 8 implements an incremental GC. With the new GC in place you should see higher ActionScript performance and less peak memory usage. I can't give you exact number since it depends on the application, but in some very special cases we have seen memory usage decrease of 40%.

The true challenge of this project was to take the existing reference counting system and move it over to the mark and sweep model in the existing code base without breaking old content. I do not know if something like this had ever been attempted before, but I amazed how well it works now. I am also really excited that the Flash Player is growing up and getting closer to other run times out there like Java.