Re: [ANNOUNCE] Git v2.19.0-rc0

[Jeff King <peff@peff.net>]

On Tue, Aug 21, 2018 at 04:41:02PM -0400, Derrick Stolee wrote:

> On 8/20/2018 6:13 PM, Junio C Hamano wrote:
> > An early preview release Git v2.19.0-rc0 is now available for
> > testing at the usual places.
> 
> As part of testing the release candidate, I ran the performance suite
> against a fresh clone of the Linux repository using v2.18.0 and v2.19.0-rc0
> (also: GIT_PERF_REPEAT_COUNT=10).

Wow, you're a glutton for punishment. :)

> I found a few nice improvements, but I
> also found a possible regression in tree walking. I say "tree walking"
> because it was revealed using p0001-rev-list.sh, but only with the
> "--objects" flag. I also saw some similar numbers on 'git log --raw'.
> 
> Test v2.18.0             v2.19.0-rc0
> --------------------------------------------------------------------------------------------
> 0001.1: rev-list --all 6.69(6.33+0.35)     6.52(6.20+0.31) -2.5%
> 0001.2: rev-list --all --objects 52.14(47.43+1.02)   57.15(51.09+1.18) +9.6%
> 
> To me, 9.6% seems out of the range of just noise for this length of a
> command, but I could be wrong. Could anyone else try to repro these results?

I got:

0001.2: rev-list --all --objects  37.07(36.62+0.45)   39.11(38.58+0.51) +5.5%

Less change, but my overall times were smaller, too, so clearly our
hardware or exact repos are a little bit different. Those numbers seem
pretty consistent in further runs.

It bisects to 509f6f62a4 (cache: update object ID functions for
the_hash_algo, 2018-07-16). Which make sense. An "--objects" traversal
spends a huge amount of time checking each tree entry to see if we've
processed that object yet, which ends up as hashcmp() in the hash table.
I expect that a fixed 20-byte memcmp() can be optimized a lot more than
one with an arbitrary value.

Even if _we_ know the value can only take on one of a few values, I
don't know that we have an easy way to tell the compiler that. Possibly
we could improve things by jumping directly to an optimized code path.
Sort of a poor-man's JIT. ;)

Doing this:

diff --git a/cache.h b/cache.h
index b1fd3d58ab..9c004a26c9 100644
--- a/cache.h
+++ b/cache.h
@@ -1023,7 +1023,10 @@ extern const struct object_id null_oid;
 
 static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {
-	return memcmp(sha1, sha2, the_hash_algo->rawsz);
+	if (the_hash_algo->rawsz == 20)
+		return memcmp(sha1, sha2, 20);
+	else
+		return memcmp(sha1, sha1, the_hash_algo->rawsz);
 }
 
 static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)
on top of v2.19-rc0 seems to give me about a 3% speedup (though I might
be imaging it, as there's a bit of noise). A function pointer in
the_hash_algo might make even more sense.

-Peff