Javascript:SpiderMonkey:ProjectGenerationGarbageCollection

Wiki conversion of original Google Document: https://docs.google.com/a/jamsni.com/document/d/1-GZ8F0ZabvdpCnRQaecId0yuu4QFYr5pAvnRC_KFchc/edit

Objective

Implement Generational Garbage collection in Spider Monkey. <Some goal based off V8 performance delta % on Earley Boyer benchmark>

• Accountable: Naveed

• Responsible: Terrence, Steve, Jon, Nicholas, and evilpie Ms2ger

• Consulted: bhackett billm

• Informed: Product Marketing

Generational GC - bug 619558

Steps

1. Exact Rooting done in JS Shell bug 753203 - 12 weeks started
   • Rooting Analysis on TinderBox (sfink) - done
     • Still need to unhide it for inbound - get it green, keep it green
   • Get the root analysis build green - two weeks?
     • Get jit-tests green bug 745742
     • Get js ref tests green
     • Get jsapi-tests green bug 831376
   • bug 773746
     • Rely on static analysis to make this not be a whack-a-mole game
2. Remove E4X - done
3. Do something about JSD
4. Add exact roots to stack structures - 2 weeks
5. AddRoot/RemoveRoot for Heap structures - 4 weeks
6. Static code analysis bug 831409
7. Fix all discovered rooting hazards (~800) (sfink,jonco) - 4 weeks
8. Optimize all discovered over-rooting (~100)
9. Automate static analysis (sfink)
10. Need a server (dm-sixgill01?)
    • http://people.mozilla.org/~bhackett/rootingHazards.html
    • http://people.mozilla.org/~bhackett/gcFunctions.html
11. JIT Integration with post barriers - 4 weeks, parallelized
    • IonMonkey bug 831506 - 1 week (bhackett)
    • JaegerMonkey bug 764876 - 2 days (bhackett)
    • Baseline JIT bug 831507 - <ask jandem/djvj for an estimate>
12. Generation Garbage Collection in the Shell
    • Implement prototype algorithm (terrence) - 1 week
    • This is a potential milestone: we would need to build it and test it on TBPL similar to how |r| works now -- |GGCJS|.
13. Exactly Root the Browser bug 831379 - 10 weeks
    • This is a potential milestone: we would turn on exact rooting for release FF at this point. We would not get a performance boost from this (necessarily), but it would lock in our work to this point.
14. Performance Tuning - 4 weeks (This should probably start asap)
    • Test against V8 Earley-Boyer benchmark.
    • Also v8 deltablue, raytrace
15. Implement a Nursery bug 706885 - 1 week
    • We may need to Implement Bill’s Pools/Zones idea to get the nursery to the requisite perf - 6 weeks
16. Refactor code to avoid rooting on hot paths and keep rooter overhead acceptable - ??? weeks (start early) (bhackett)
17. Re-implement HashTable rekeying
    • We undid this code because it was a perf regression and was still a bit buggy. (terrence) - 3 weeks
18. Make the Post Barrier Verifier Green in the browser bug 764882
19. Investigate how long it will take to do generational barriers - 1 week
    • We may need to rewrite the maps in xpconnect and the browser in terms of HashTable: this could be a bunch of work.
20. Make JS_IsAboutToBeFinalized indirect bug 765432 - 10 weeks

Other

Should we establish a new benchmark specifically for GGC. (sfink votes yes)

What would it measure? there are multiple goals, e.g. throughput/MMU/pause time

Should we make it a public benchmark?

What workloads should we consider?

compartmental GC very important to us, not necessarily applicable to other implementations allocation rate (broken down into live vs garbage), steady behavior vs swapping between allocation + computation modes, etc.

How can we keep our advantage on Splay benchmark possible to do with TI --- look at types of objects promoted from nursery to major heap, eventually start allocating them directly in the major heap

Compacting GC - Revisit in Febuary/March - 1 Month Delayed until exact rooting is fully done. We can add this in if we have time in the schedule that we cannot parallelize.

Risks

• Team is responsible for many top crashers

• External rooting API has not been designed

• GGC algorithm has not been decided on - it may not be faster

• JSD1 exact rooting is lurking