Re: Make GIT_USE_LOOKUP default?

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 18, 2013, 16:44 UTC
Message-ID: <7vtxo8ody3.fsf@alter.siamese.dyndns.org>
In-Reply-To: <20130318073229.GA5551@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 27 quoted lines

> By the way, looking at that made me think for a few minutes about
> hashcmp, and I was surprised to find that we use an open-coded
> comparison loop. That dates back to this thread by Ingo:
>
>   http://article.gmane.org/gmane.comp.version-control.git/172286
>
> I could not replicate his benchmarks at all. In fact, my measurements
> showed a slight slowdown with 1a812f3 (hashcmp(): inline memcmp() by
> hand to optimize, 2011-04-28).
>
> Here are my best-of-five numbers for running "git rev-list --objects
> --all >/dev/null" on linux-2.6.git:
>
>   [current master, compiled with -O2]
>   real    0m45.612s
>   user    0m45.140s
>   sys     0m0.300s
> ...
>   [revert 1a812f3 (i.e., go back to memcmp), -O2]
>   real    0m44.358s
>   user    0m43.876s
>   sys     0m0.316s
> ...
> I wonder why we get such different numbers. Ingo said his tests are on a
> Nehalem CPU, as are mine (mine is an i7-840QM). I wonder if we should be
> wrapping the optimization in an #ifdef, but I'm not sure which flag we
> should be checking.

FWIW, I am getting something like this on my

$ grep 'model name' /proc/cpuinfo | uniq -c
  4 model name      : Intel(R) Core(TM)2 Quad  CPU   Q9450  @ 2.66GHz

The same "rev-list --objects --all >/dev/null", best of five:

[current master, compiled with -O2] real 0m39.956s user 0m39.562s sys 0m0.396s

[revert 1a812f3 (i.e., go back to memcmp), -O2] real 0m42.161s user 0m41.879s sys 0m0.284s

It could be that the difference may be how well memcmp() is optimized, no? My box runs Debian with libc6 2.11.3-4 and gcc 4.4.5.