Enhancing Garbage Collection Synchronization in Multi-Threaded Applications

1. Removing GC Synchronisation Andy C. King and Richard Jones University of Kent at Canterbury

2. Overview • The Problem • An Observation • L & S Heaplets • Implementation • A Fly in the Ointment • Swatting the Fly • Measurements • Future Work

3. The Problem • Concurrency in multi-threaded applications • Synchronisation is necessary • Language level • synchronized • VM level • mutator and garbage collector (GC) threads • suspending threads before collecting • the latter can be quite costly

4. An Observation • Objects may be thread-local (L) • Reachable only within their allocating thread • do not need synchronisation for such objects • Can manage these objects locally • Place in per-thread heap regions called heaplets • No need to stop other threads to collect such objects • Shared objects (G) go in global heap

5. L & G Heaplets T1 G L1 T2 L2

6. Implementation • Variation on Steensgaard [Stee00] • Escape analysis technique • automatically determine local objects • mark their allocation sites • patch to use appropriate heaplet • Flow-insensitive • Context-sensitive (specialisation) • performed at runtime

7. Implementation (2) • Operates on a snapshot of the world • set of classes loaded at that point in time • JDK 1.22_06 Production Release for Solaris • new GC with heaplets • support specialisation • patches applied in interpreter • patched/specialised methods can be JIT’ed

8. A Fly in the Ointment • Dynamic class loading • Class loaded after snapshot • May override method containing L allocation site • Shares the object • L allocation site now S • (transitive closure)

9. A Fly in the Ointment (2) • 1 classA { • 2 publicvoid do(Object o) { } • 3 } • 4 classMain { • 5 publicstaticvoid main(String[] args) { • 6 A a = newA(); • 7 foo(a); • SNAPSHOT ANALYSIS • 30 A a’ = newA’(); LOAD CLASS A’ • 31 foo(a’); • 32 } • 33 publicstaticvoid foo(A inA) { • 34 Object o = newObject(); • 35 inA.do(o); • 36 } • 37 }

10. Swatting the Fly • Truly shared (G) • reachable from >1 thread • G heap • Truly local (L) • reachable from exactly 1 thread • can prove type • L heaplet • Optimistically local (OL) • reachable from 1 thread • cannot prove type • OL heaplet

11. Swatting the Fly (2) T1 G OL1 L1 T2 OL2 L2

12. Swatting the Fly (3) • Analyse new classes immediately • Compare methods • If mismatched, OL sites are broken • Class is non-conforming • OL heaplet becomes G • can be done very cheaply • only OL heaplets of referring threads are affected

13. Swatting the Fly (4) T1 G OL1 L1 T2 OL2 L2

14. Measurements

15. Measurements (2)

16. Future Work • Finally get the bloody numbers from the GC…

17. Q&A Andy C. King and Richard Jones University of Kent at Canterbury