David Mandelin's blog » Mozilla JavaScript 2011

David Mandelin's blog » Mozilla JavaScript 2011

The reason for the pauses is that SpiderMonkey uses an old-school stop-the-world mark-and-sweep collector. Briefly, it works like this:

1. Based on some heuristics, the JS engine decides it is time to collect some garbage.
2. The GC finds all the GC roots, which are the immediately accessible objects: JS local variables, the JS global object, JS objects stored on the C++ stack, and a few other things.
3. The GC marks all objects that can be reached from the roots, by following all the pointers stored in the roots, then all the pointers stored in the objects reached from the roots, and so on.
4. The GC sweeps over all allocated objects. If an object is not marked, there is no way for the program to access it, so it can never be used again, and the GC frees it.
The main problem with stop-the-world mark-and-sweep GC is that if there are a lot of live objects, it can take a long time to mark all those objects. “A long time” typically means 100 milliseconds, which is not that long, but is disruptive to animation and is noticeably jerky.

Our first step in fixing GC pauses will be incremental GC. Incremental GC means that instead of stopping the program to mark everything, the GC periodically pauses the program to do a little bit of marking, say 3 milliseconds worth. There is an overhead to starting and stopping a mark phase, so the shorter the pause time, the slower the actual program runs. But we think we can make the pause time unnoticeable without having too much impact on throughput.

Note that the sweep phase can also take a long time, so we’ll need to do some work there also, such as sweeping incrementally or in concurrently on a different thread.

The longer-term goal is to move to generational GC. It’s more complicated than incremental GC, so I won’t go into details now, but the key benefits of generational GC are that (a) it is very fast at collecting short-lived objects (technically, it actually manages to collect them without looking at them or doing anything to them at all), and (b) it helps make creating objects faster.

Bill McCloskey and Chris Leary are working on the new GCs. Gregor Wagner has also been working independently on specific improvements, like doing more sweeping off the main thread.