Re: [PATCH v2 3/3] connected: implement connectivity check using bitmaps

From: Jeff King <peff@peff.net>
Date: Jun 30, 2021, 02:04 UTC
Message-ID: <YNvRSJI1oWN9W25Y@coredump.intra.peff.net>
In-Reply-To: <YNuiM8TR5evSeNsN@nand.local>

On Tue, Jun 29, 2021 at 06:44:03PM -0400, Taylor Blau wrote:

Show 16 quoted lines

>     $ hyperfine \
>       'GIT_READ_COMMIT_TABLE=0 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2' \
>       'GIT_READ_COMMIT_TABLE=1 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2' \
>       --prepare='sync; echo 3 | sudo tee /proc/sys/vm/drop_caches'
> 
> 		Benchmark #1: GIT_READ_COMMIT_TABLE=0 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2
> 			Time (mean ± σ):     141.1 ms ±   2.5 ms    [User: 13.0 ms, System: 64.3 ms]
> 			Range (min … max):   136.2 ms … 143.4 ms    10 runs
> 
> 		Benchmark #2: GIT_READ_COMMIT_TABLE=1 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2
> 			Time (mean ± σ):      28.7 ms ±   3.2 ms    [User: 6.5 ms, System: 10.0 ms]
> 			Range (min … max):    22.0 ms …  31.0 ms    21 runs
> 
> 		Summary
> 			'GIT_READ_COMMIT_TABLE=1 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2' ran
> 				4.91 ± 0.55 times faster than 'GIT_READ_COMMIT_TABLE=0 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2'

I was curious why your machine is so much slower than mine. With the current bitmap format, I can run that command pretty consistently in ~22ms. But I think the trick here is the cache-dropping. The cold-cache performance is going to be very dependent on faulting in the extra bytes (and you can see that the actual CPU time in the first case is much smaller than the runtime, so it really is waiting on the disk).

In the warm-cache case, the improvement seems to go away (or maybe I'm holding it wrong; even in the cold-cache case I don't get anywhere near as impressive a speedup as you showed above). Which isn't to say that cold-cache isn't sometimes important, and this may still be worth doing. But it really seems like the CPU involve in walking over the file isn't actually that much.

I got an even more curious result when adding in "--not --all" (which the connectivity check under discussion would do). There the improvement from your patch should be even less, because we'd end up reading most of the bitmaps anyway. But I got:

  $ hyperfine \
       'GIT_READ_COMMIT_TABLE=0 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2 --not --all' \
       'GIT_READ_COMMIT_TABLE=1 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2 --not --all' \
       --prepare='sync; echo 3 | sudo tee /proc/sys/vm/drop_caches'

  Benchmark #1: GIT_READ_COMMIT_TABLE=0 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2 --not --all
    Time (mean ± σ):      4.197 s ±  0.823 s    [User: 284.7 ms, System: 734.5 ms]
    Range (min … max):    2.612 s …  5.009 s    10 runs
   
  Benchmark #2: GIT_READ_COMMIT_TABLE=1 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2 --not --all
    Time (mean ± σ):      4.498 s ±  0.612 s    [User: 315.3 ms, System: 829.7 ms]
    Range (min … max):    3.010 s …  5.072 s    10 runs
   
  Summary
    'GIT_READ_COMMIT_TABLE=0 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2 --not --all' ran
      1.07 ± 0.26 times faster than 'GIT_READ_COMMIT_TABLE=1 git.compile rev-list --count --objects --use-bitmap-index 2ab38c17aac10bf55ab3efde4c4db3893d8691d2 --not --all'

which was actually faster _without_ the extra table. 7% isn't a lot, especially for a cold-cache test, so that may just be noise. But it doesn't seem like a clear win to me.

-Peff