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..
René
