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Re: [PATCH v2 3/3] grep: disable threading in all but worktree case

From
Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date
Dec 24, 2011, 01:39 UTC
Message-ID
<CACBZZX67WhcdhXdqOm8gZHW7C3YMbV2KzeytwjHwsnF=8-M_+w@mail.gmail.com>
In-Reply-To
<87mxaihpiq.fsf@thomas.inf.ethz.ch>
2011/12/23 Thomas Rast <trast@student.ethz.ch>:
Show 19 quoted lines
> Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:
>
>> On Fri, Dec 2, 2011 at 14:07, Thomas Rast <trast@student.ethz.ch> wrote:
>>
>>> I conjecture that this is caused by contention on
>>> read_sha1_mutex. [...] So disable threading entirely when not
>>> scanning the worktree
>>
>> Why does git-grep even need to keep a mutex to call read_sha1_file()?
>> It's inherently a read-only operation isn't it? If the lock is needed
>> because data is being shared between threads in sha1_file.c shouldn't
>> we tackle that instead of completely disabling threading?
>
> The problem is that all sorts of data is shared.  See
>
>  http://thread.gmane.org/gmane.comp.version-control.git/186618
>
> But I need to go through it again, there are some races and double locks
> in the posted version.
I mentioned this on IRC, but I thought I'd bring it up here too.

Is the expensive part of git-grep all the setup work, or the actual traversal and searching? I'm guessing it's the latter.

In that case an easy way to do git-grep in parallel would be to simply spawn multiple sub-processes, e.g. if we had 1000 files and 4 cores:

 1. Split the 1000 into 4 parts 250 each.
 2. Spawn 4 processes as: git grep <pattern> -- <250 files>
 3. Aggregate all of the results in the parent process
Previous: Thomas RastNext: Jeff King
Message 25 of 35 in “grep multithreading and scaling”
  1. 0/3 grep multithreading and scalingThomas Rast, Dec 2, 2011
  2. 1/3 grep: load funcname patterns for -WThomas Rast, Dec 2, 2011
  3. 2/3 grep: enable threading with -p and -W using lazy attribute lookupThomas Rast, Dec 2, 2011
  4. 3/3 grep: disable threading in all but worktree caseThomas Rast, Dec 2, 2011
  5. René ScharfeDec 2, 2011
  6. Thomas RastDec 5, 2011
  7. René ScharfeDec 6, 2011
  8. 4/2 grep: turn off threading for non-worktreeRené Scharfe, Dec 6, 2011
  9. Jeff KingDec 7, 2011
  10. René ScharfeDec 7, 2011
  11. Jeff KingDec 7, 2011
  12. J. Bruce FieldsDec 7, 2011
  13. Jeff KingDec 7, 2011
  14. Thomas RastDec 7, 2011
  15. René ScharfeDec 7, 2011
  16. Pete WyckoffDec 10, 2011
  17. René ScharfeDec 12, 2011
  18. Jeff KingDec 7, 2011
  19. René ScharfeDec 7, 2011
  20. Jeff KingDec 7, 2011
  21. Thomas RastDec 7, 2011
  22. René ScharfeDec 7, 2011
  23. Ævar Arnfjörð BjarmasonDec 23, 2011
  24. Thomas RastDec 23, 2011
  25. Ævar Arnfjörð BjarmasonDec 24, 2011
  26. Jeff KingDec 24, 2011
  27. Nguyen Thai Ngoc DuyDec 24, 2011
  28. Nguyen Thai Ngoc DuyDec 24, 2011
  29. Jeff KingDec 24, 2011
  30. Nguyen Thai Ngoc DuyDec 25, 2011
  31. Jeff KingDec 2, 2011
  32. Thomas RastDec 5, 2011
  33. Thomas RastDec 5, 2011
  34. Jeff KingDec 6, 2011
  35. Eric HermanDec 2, 2011

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