Myths & Realities The Performance Impact of Garbage Collection

1. Myths & RealitiesThe Performance Impact of Garbage Collection

2. Background • No prior apples-to-apples comparisons • MMTk • Canonical policies implemented (SS, MS, RC, genX, etc) • Shared mechanisms • Good performance (match/beat old Watson GCs) • Ideal platform for apples-to-apples comparisons Myths & Realities: The performance impact of garbage collection

3. Some Questions • Architecture • How well do modern OO languages play to modern architectures? • Collection • Is generational GC “a waste of time”? • Are write barriers expensive? • Allocation • Free list or bump pointer? • “Locality is everything” • Really??? • Is it different for young & old? Why? • Locality and architecture • What is the impact, what is the trend? Myths & Realities: The performance impact of garbage collection

4. Methodology • Jikes RVM & MMTk • Platforms • 1.6GHz G5 (PowerPC 970) • 1.9GHz AMD Athlon 2600+ • 2.6GHz Intel P4 • Linux 2.6.0 with perfctr patch & libraries • Separate accounting of GC & Mutator perf counts • SPECjvm98 & pseudojbb Myths & Realities: The performance impact of garbage collection

5. Architecture Myths & Realities: The performance impact of garbage collection

6. Relative Performance Myths & Realities: The performance impact of garbage collection

7. Architecture - Q & A How big is the mismatch between modern arch & modern languages??? Myths & Realities: The performance impact of garbage collection

8. Allocation Myths & Realities: The performance impact of garbage collection

9. Allocation Choices • Bump pointer • ~70 bytes IA32 instructions, 726MB/s • Free list • ~140 bytes IA32 instructions, 654MB/s • Bump pointer 11% faster in tight loop • < 1% in practical setting • No significant difference (?) • Second order effects? • Locality?? • Collection mechanism?? Myths & Realities: The performance impact of garbage collection

10. Implications for Locality • Compare SS & MS mutator • Mutator time = total – GC time • Mutator memory performance: L1, L2 & TLB Myths & Realities: The performance impact of garbage collection

11. jess Myths & Realities: The performance impact of garbage collection

12. jess Myths & Realities: The performance impact of garbage collection

13. jess Myths & Realities: The performance impact of garbage collection

14. jess Myths & Realities: The performance impact of garbage collection

15. javac Myths & Realities: The performance impact of garbage collection

16. pseudojbb Myths & Realities: The performance impact of garbage collection

17. db Myths & Realities: The performance impact of garbage collection

18. Locality Myths & Realities: The performance impact of garbage collection

19. Bump Pointer & Free List • Is the locality differential age-dependant? • Re-run experiment with GenCopy & GenMS • Generational variants of MarkSweep & SemiSpace • Young objects treated identically • Mature objects either SemiSpace or MarkSweep Myths & Realities: The performance impact of garbage collection

20. Bump Pointer & Free List Myths & Realities: The performance impact of garbage collection

21. Bump Pointer & Free List • Why? • Mature space locality? • Nursery absorbs most allocs – lower frag • Relatively frequent copying in SS • Contigious allocation in nursery? Myths & Realities: The performance impact of garbage collection

22. Bump Pointer & Free List • Why? • Mature space locality? • Nursery absorbs most allocs – lower frag • Relatively frequent copying in SS • Contigious allocation in nursery? Myths & Realities: The performance impact of garbage collection

23. Bump Pointer & Free List • Why? • Mature space locality • Nursery absorbs most allocs – lower frag • Relatively frequent copying in SS • Contigious allocation in nursery Myths & Realities: The performance impact of garbage collection

24. Bump Pointer & Free List Run SS & MS in “infinite” heap Myths & Realities: The performance impact of garbage collection

25. Bump Pointer & Free List Run SS & MS in “infinite” heap Myths & Realities: The performance impact of garbage collection

26. Bump Pointer & Free List Run SS & MS in “infinite” heap • Infinite heap does not degrade locality (!?) • Exceptions: jess (degrades), db (improves) why? • Is spatial locality unimportant in mature space??? Myths & Realities: The performance impact of garbage collection

27. BP & FL Locality Implications • Is spatial locality unimportant in mature space?? • No [Huang et al OOPSLA 2004] • But perhaps temporal locality is more significant • Seems clear contiguous allocation is good • Vast majority of objects < cache line • h/w prefetcher may be significant • Hard to improve over alloc order, easy to mess up? • Unlikely to be true: MarkSweep < Compacting < SemiSpace Myths & Realities: The performance impact of garbage collection

28. Locality &Architecture Myths & Realities: The performance impact of garbage collection

29. MS/SS Crossover: 1.6GHz PPC Myths & Realities: The performance impact of garbage collection

30. MS/SS Crossover: 1.9GHz AMD Myths & Realities: The performance impact of garbage collection

31. MS/SS Crossover: 2.6GHz P4 Myths & Realities: The performance impact of garbage collection

32. MS/SS Crossover: 3.2GHz P4 Myths & Realities: The performance impact of garbage collection

33. MS/SS Crossover locality space 2.6GHz 1.6GHz 1.9GHz 3.2GHz Myths & Realities: The performance impact of garbage collection

34. Conclusions • Need for (re) evaluation of GC performance • Key GC insights > 20yrs old • Technology has changed • Absence of apples-to-apples comparisons • Highly architecturally sensitive • MMTk + perf counters • High performance infrastructure • Multiple GCs, shared mechanisms • Some myths exposed & new realities Myths & Realities: The performance impact of garbage collection