From: Jeff King <peff@peff.net>
To: "brian m. carlson" <sandals@crustytoothpaste.net>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
"Stephen Smith" <ischis2@cox.net>, git <git@vger.kernel.org>
Subject: Re: SHA-256 transition
Date: Fri, 24 Jun 2022 06:52:36 -0400 [thread overview]
Message-ID: <YrWXdNGZGN7gXL40@coredump.intra.peff.net> (raw)
In-Reply-To: <YrI9dvfoc5NYgVDq@tapette.crustytoothpaste.net>
On Wed, Jun 22, 2022 at 12:29:59AM +0000, brian m. carlson wrote:
> > We've since migrated our default hash function from SHA-1 to SHA-1DC
> > (except on vanilla OSX, see [2]). It's a variant SHA-1 that detects the
> > SHAttered attack implemented by the same researchers. I'm not aware of a
> > current viable SHA-1 collision against the variant of SHA-1 that we
> > actually use these days.
>
> That's true, but that still doesn't let you store the data. There is
> some data that you can't store in a SHA-1 repository, and SHA-1DC is
> extremely slow. Using SHA-256 can make things like indexing packs
> substantially faster.
I'm curious if you have numbers on this. I naively converted linux.git
to sha256 by doing "fast-export | fast-import" (the latter in a sha256
repo, of course, and then both repacked with "-f --window=250" to get
reasonable apples-to-apples packs).
Running "index-pack --verify" on the result takes about the same time
(this is on an 8-core system, hence the real/user differences):
[sha1dc]
real 2m43.754s
user 10m52.452s
sys 0m36.745s
[sha256]
real 2m41.884s
user 12m23.344s
sys 0m35.222s
The sha256 repo actually has about 10% fewer objects (I didn't
investigate, but this is perhaps due to cutting out tags and a few other
things to convince fast-export to finish running). I'm not sure about
the extra user time (multicore timings here are funny because of
frequency scaling, so I think the "real" line is more interesting). So
sha256 actually comes out a bit worse here. On the other hand, this is
just using our blk_SHA256 implementation. There may be faster
alternatives (including ones with hardware support).
I wouldn't be at all surprised if the difference isn't substantial in
the long run, though. The repo is on the order of 100GB of object data.
That's a lot to hash, but it's also just a lot to deal with at all (zlib
inflating, applying deltas, etc).
Anyway, this is a pretty rough cut at an experiment. I was mostly
curious if you had done something more advanced, and/or gotten different
results.
-Peff
next prev parent reply other threads:[~2022-06-24 10:53 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-20 22:51 SHA-256 transition Stephen Smith
2022-06-20 23:13 ` rsbecker
2022-06-21 10:25 ` Ævar Arnfjörð Bjarmason
2022-06-21 13:18 ` rsbecker
2022-06-21 18:14 ` Ævar Arnfjörð Bjarmason
2022-06-22 0:29 ` brian m. carlson
2022-06-23 0:45 ` Stephen Smith
2022-06-23 1:44 ` brian m. carlson
2022-06-23 15:32 ` Junio C Hamano
2022-06-23 22:21 ` Ævar Arnfjörð Bjarmason
2022-06-24 0:29 ` Kyle Meyer
2022-06-24 1:03 ` Stephen Smith
2022-06-24 1:19 ` Ævar Arnfjörð Bjarmason
2022-06-24 14:42 ` Jonathan Corbet
2022-06-24 10:52 ` Jeff King [this message]
2022-06-24 15:49 ` Ævar Arnfjörð Bjarmason
2022-06-25 8:53 ` brian m. carlson
2022-06-26 0:09 ` Plan for SHA-256 repos to support SHA-1? Eric W. Biederman
2022-06-26 0:27 ` Junio C Hamano
2022-06-26 15:19 ` brian m. carlson
2022-07-01 18:00 ` SHA-256 transition Jeff King
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