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Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons

From
IMIngo Molnar <mingo@elte.hu>
Date
Apr 28, 2011, 20:32 UTC
Message-ID
<20110428203205.GD24755@elte.hu>
In-Reply-To
<BANLkTim90XOLRBPtVCXFb1ptkmUvHGRqeg@mail.gmail.com>
* Erik Faye-Lund <kusmabite@gmail.com> wrote:
Show 6 quoted lines
> Thanks. I also timed on my end (on Windows), and I came to the same 
> conclusion (but the improvements of your original was somewhat smaller in my 
> end; could be due to the test-case). It seems like the early-out wasn't the 
> only reason your original patch performed faster. It could be that memcmp 
> (probably) didn't get inlined, and the extra function call outweighs the 
> complexity. [...]

Function calls arent that heavy really. My measurements identified the following effects:

 - profiling of stalled cycles clearly pinpointed the REP MOV string 
   instruction.
 - the patched code had less branch-misses - the clearer and inlined open-coded 
   loop is probably easier for the CPU to speculate along - while REP MOV 
   string ops are 'opaque' and the result might be harder to speculate.

So i think the main benefit of my patch is that it avoids the REP MOV instruction.

Thanks,
	Ingo
Previous: Erik Faye-LundNext: Alex Riesen
Message 29 of 45 in “git gc: Speed it up by 18% via faster hash comparisons”
  1. git gc: Speed it up by 18% via faster hash comparisonsIngo Molnar, Apr 27, 2011
  2. Ingo MolnarApr 27, 2011
  3. Jonathan NiederApr 27, 2011
  4. Ingo MolnarApr 28, 2011
  5. Jonathan NiederApr 28, 2011
  6. Ingo MolnarApr 28, 2011
  7. Dmitry PotapovApr 28, 2011
  8. Junio C HamanoApr 27, 2011
  9. Ralf BaechleApr 28, 2011
  10. Bernhard R. LinkApr 28, 2011
  11. Andreas EricssonApr 28, 2011
  12. Erik Faye-LundApr 28, 2011
  13. H. Peter AnvinApr 28, 2011
  14. Ingo MolnarApr 28, 2011
  15. Erik Faye-LundApr 28, 2011
  16. Ingo MolnarApr 28, 2011
  17. Ingo MolnarApr 28, 2011
  18. Erik Faye-LundApr 28, 2011
  19. Pekka EnbergApr 28, 2011
  20. Erik Faye-LundApr 28, 2011
  21. Pekka EnbergApr 28, 2011
  22. Erik Faye-LundApr 28, 2011
  23. Pekka EnbergApr 28, 2011
  24. Jonathan NiederApr 28, 2011
  25. Erik Faye-LundApr 28, 2011
  26. Ingo MolnarApr 28, 2011
  27. Ingo MolnarApr 28, 2011
  28. Erik Faye-LundApr 28, 2011
  29. Ingo MolnarApr 28, 2011
  30. Alex RiesenApr 29, 2011
  31. H. Peter AnvinApr 29, 2011
  32. Tor ArntsenApr 28, 2011
  33. H. Peter AnvinApr 28, 2011
  34. Andreas EricssonApr 28, 2011
  35. Erik Faye-LundApr 28, 2011
  36. Ingo MolnarApr 28, 2011
  37. Nguyen Thai Ngoc DuyApr 28, 2011
  38. Erik Faye-LundApr 28, 2011
  39. Junio C HamanoApr 28, 2011
  40. Dmitry PotapovApr 28, 2011
  41. Dmitry PotapovApr 28, 2011
  42. Ingo MolnarApr 28, 2011
  43. Dmitry PotapovApr 28, 2011
  44. Ingo MolnarApr 28, 2011
  45. Ingo MolnarApr 28, 2011

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