Re: [PATCH] midx: use buffered I/O to talk to pack-objects

[Derrick Stolee <stolee@gmail.com>]

On 8/11/2020 12:08 PM, René Scharfe wrote:
> Am 03.08.20 um 14:39 schrieb Derrick Stolee:
>> On 8/2/2020 10:38 AM, René Scharfe wrote:
>>> Like f0bca72dc77 (send-pack: use buffered I/O to talk to pack-objects,
>>> 2016-06-08), significantly reduce the number of system calls and
>>> simplify the code for sending object IDs to pack-objects by using
>>> stdio's buffering and handling errors after the loop.
>>
>> Good find. Thanks for doing this important cleanup.
>>
>> Outside of Chris's other feedback, this looks like an obviously
>> correct transformation.
> 
> I spent a surprising amount of time trying to find a solution that is
> easy to use and allows precise error handling.  But now I get second
> thoughts.  The main selling point of buffering is better performance,
> which is achieved by reducing the number of system calls.  How much
> better actually?
> 
> So I get this in my Git repo clone without this patch:
> 
>   $ strace --summary-only --trace=write git multi-pack-index repack --no-progress
>   % time     seconds  usecs/call     calls    errors syscall
>   ------ ----------- ----------- --------- --------- ----------------
>   100.00    2.237478           2    921650           write
>   ------ ----------- ----------- --------- --------- ----------------
>   100.00    2.237478                921650           total
> 
> And here's the same with the patch:
> 
>   % time     seconds  usecs/call     calls    errors syscall
>   ------ ----------- ----------- --------- --------- ----------------
>   100.00    0.013293           2      4613           write
>   ------ ----------- ----------- --------- --------- ----------------
>   100.00    0.013293                  4613           total
> 
> Awesome, right?  write(2) calls are down by a factor of almost 200 and
> the time spent on them is reduced significantly, as advertised.  Let's
> ask hyperfine for a second opinion though.  Without this patch:
> 
>   Benchmark #1: git multi-pack-index repack --no-progress
>     Time (mean ± σ):      1.652 s ±  0.206 s    [User: 1.383 s, System: 0.317 s]
>     Range (min … max):    1.426 s …  1.890 s    10 runs
> 
> And the same with this patch applied:
> 
>     Time (mean ± σ):      1.635 s ±  0.199 s    [User: 1.363 s, System: 0.204 s]
>     Range (min … max):    1.430 s …  1.871 s    10 runs
> 
> OK, so system time is down by ca. 50%, but the total duration is
> basically unchanged.  It seems strace added quite some overhead to our
> measurement above.
> 
> Anyway, now I wonder if adding our own buffer on top if the
> OS-internal pipe buffer is actually worth it.  The numbers above are
> from Debian testing , by the way.  Perhaps buffering still pays off on
> operating systems with slower pipes..

For what it's worth, I took your patch and applied it on Git for Windows
and tested 'git multi-pack-index repack' on my copy of the Git repo
(which includes Git for Windows and microsoft/git for a total of
1.7 million objects) and saw the time improve from 22.3s to 16.6s!

The "Enumerating objects" progress bar was visibly faster when I was
watching the command.

I was not expecting such a huge speed bump, seeing how the objects
are being repacked, so this command includes complicated processes
like delta compression an zlib compression.

Thanks! This is definitely worth the speed boost on Windows.

-Stolee