Re: Which hash function to use, was Re: RFC: Another proposed hash function transition plan

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Jun 16, 2017, 21:39 UTC
Message-ID: <87tw3f8vez.fsf@gmail.com>
In-Reply-To: <20170616212414.GC133952@aiede.mtv.corp.google.com>

On Fri, Jun 16 2017, Jonathan Nieder jotted:

Show 6 quoted lines

> Part of the reason I suggested previously that it would be helpful to
> try to benchmark Git with various hash functions (which didn't go over
> well, for some reason) is that it makes these comparisons more
> concrete.  Without measuring, it is hard to get a sense of the
> distribution of input sizes and how much practical effect the
> differences we are talking about have.

It would be great to have such benchmarks (I probably missed the "didn't go over well" part), but FWIW you can get pretty close to this right now in git by running various t/perf benchmarks with BLKSHA1/OPENSSL/SHA1DC.

Between the three of those (particularly SHA1DC being slower than OpenSSL) you get a similar performance difference as some SHA-1 v.s. SHA-256 benchmarks I've seen, so to the extent that we have existing performance tests it's revealing to see what's slower & faster.

It makes a particularly big difference for e.g. p3400-rebase.sh.