Re: x86 SHA1: Faster than OpenSSL

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sun, 2 Aug 2009, George Spelvin wrote:
> 
> The original code was excellent, but it was optimized when the P4 was new.
> After a bit of tweaking, I've inflicted a slight (1.4%) slowdown on the
> P4, but a small-but-noticeable speedup on a variety of other processors.
> 
> Before      After       Gain    Processor
> 1.585248    1.353314	+17%	2500 MHz Phenom
> 3.249614    3.295619	-1.4%	1594 MHz P4
> 1.414512    1.352843	+4.5%	2.66 GHz i7
> 3.460635    3.284221	+5.4%	1596 MHz Athlon XP
> 4.077993    3.891826	+4.8%	1144 MHz Athlon
> 1.912161    1.623212	+17%	2100 MHz Athlon 64 X2
> 2.956432    2.940210	+0.55%	1794 MHz Mobile Celeron (fam 15 model 2)

It would be better to have a more git-centric benchmark that actually 
shows some real git load, rather than a sha1-only microbenchmark.

The thing that I'd prefer is simply

	git fsck --full

on the Linux kernel archive. For me (with a fast machine), it takes about 
4m30s with the OpenSSL SHA1, and takes 6m40s with the Mozilla SHA1 (ie 
using a NO_OPENSSL=1 build).

So that's an example of a load that is actually very sensitive to SHA1 
performance (more so than _most_ git loads, I suspect), and at the same 
time is a real git load rather than some SHA1-only microbenchmark. It also 
shows very clearly why we default to the OpenSSL version over the Mozilla 
one.

NOTE! I didn't do multiple runs to see how stable the numbers are, and 
so it's possible that I exaggerated the OpenSSL advantage over the 
Mozilla-SHA1 code. Or vice versa. My point is really only that I don't 
know how meaningful a "50 x 1M SHA1" benchmark is, while I know that a 
"git fsck" benchmark has at least _some_ real life value.

		Linus