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Data layouts for object-oriented programs

Data layouts for object-oriented programs. Martin Hirzel IBM Research SIGMETRICS 6/16/2007. Object-oriented programs put data in objects. Caches and TLBs put data in blocks. Scattering objects over blocks causes cache/TLB misses. Misses cost time. Problem. o 1. o 5. o 6. o 8. o 2. o 3.

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Data layouts for object-oriented programs

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  1. Data layouts forobject-oriented programs Martin HirzelIBM Research SIGMETRICS 6/16/2007

  2. Object-oriented programs put datain objects. Caches and TLBs put data in blocks. Scattering objects over blocks causes cache/TLB misses. Misses cost time. Problem o1 o5 o6 o8 o2 o3 o7 o10 o4 o9 o11 cache line TLB page

  3. Most object-oriented languages use garbage collection. Garbage collection can move objects. To avoid misses, move objects to the right cache/TLB blocks. Simple, right? Solution o1 o5 o6 o8 o2 o3 o7 o10 o4 o9 o11 o1 o2 o3 o4 o9 o10 o11 o5 o6 o7 o8

  4. scan=free scan scan scan scan free free free free Cheney Copying GC From-space o1 o2 o3 o4 o5 o6 o7 o8 o9 o10 To-space Copied &not yet scanned Copied & scanned

  5. BF: Breadth-first layout 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

  6. DF: Depth-first layout 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

  7. HI: Hierarchical layout 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

  8. AO: Allocation order layout 1 2d 3 4d 5d 6 7 8d 9 1c 3c 6c 7c 9c

  9. SZ: Size segregation layout 1 2 3d 4 5d 6 7 8d 9 1c 6c 2c 7c 9c 4c

  10. TH: Thread local layout 001 010 1 8 3 111 11 2 4 9 12 101 110 10 6 5 7 100

  11. Which layout is best, and which is worst? How much does it matter in practice? How similar are the layouts? How much does it matter in the limit? Problem

  12. Solutions? • Appeal to intuition • They can’t all be right! • Formal • Petrank/Rawitz showed hardness • Simulation • Who would believe those numbers? • Brute-force • Do you have a few person-years to spare?

  13. Avoiding Heisenberg Effects Garbage collector Garbage collector Garbage collector Application Application Time • Exclude garbage collector performance • Measure real effect of layout on application • Implement the layouts with simple algorithms

  14. Object sorting garbage collection populate sort copy fixup Sort keys: AO, AS, PO, RA, SZ, TH, TY

  15. 32 Benchmarks

  16. % Mutator time overhead • All layouts sometimes best, sometimes worst • RA is worst, as expected • Low averages, but beware of worst cases! • AO has best average (but not by much) • DF has most best cases • TH has most benign worst case • Performance impact increases with SMP • Conclusions for AO, DF, TH, RA still hold

  17. % Mutator miss rate increases • Layouts have large impact on miss rates • Miss rates confirm overhead conclusions • Miss rates can not replace time measurements

  18. Layout similarities and differences • AO, PO, and TH are quite similar • As expected, RA is far out

  19. Benchmarks:largest avg. overhead Baseline:best observed Linear regression: Estimated limit mutator time

  20. Related work

  21. Conclusions • Layouts matter little on average, but: • Beware of the worst cases! • Layout importance increases with SMP • All layouts are sometimes best,sometimes worst • AO has best average • DF has most best-cases • TH has best worst-cases

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