From: "brian m. carlson" <sandals@crustytoothpaste.net>
To: Jeff King <peff@peff.net>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
"Stephen Smith" <ischis2@cox.net>, git <git@vger.kernel.org>
Subject: Re: SHA-256 transition
Date: Sat, 25 Jun 2022 08:53:53 +0000 [thread overview]
Message-ID: <YrbNIUnftj+Ooumo@tapette.crustytoothpaste.net> (raw)
In-Reply-To: <YrWXdNGZGN7gXL40@coredump.intra.peff.net>
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On 2022-06-24 at 10:52:36, Jeff King wrote:
> 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).
I did the same thing, except I just did a regular gc and not a custom
repack, and I created both a SHA-1 and SHA-256 repo from the same
original.
> 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
Here are my results:
[sha256]
time ~/checkouts/git/git index-pack --verify .git/objects/pack/pack-*.pack
~/checkouts/git/git index-pack --verify .git/objects/pack/pack-*.pack 2768.42s user 181.00s system 185% cpu 26:31.70 total
[sha1dc]
time ~/checkouts/git/git index-pack --verify .git/objects/pack/pack-*.pack
~/checkouts/git/git index-pack --verify .git/objects/pack/pack-*.pack 3041.28s user 184.84s system 199% cpu 26:54.74 total
Note that in my case, I'm using an accelerated hardware-based SHA-256
implementation (Nettle, which I will send a patch for soon). This is a
brand new ThinkPad X1 Carbon Gen 10 with an i7-1280P (with 20 "cores" of
different sizes).
So this is about 9% faster in terms of total CPU usage on SHA-256 with
that implementation. The wallclock time is less impressive here.
Of course, it might be slower in software, but considering that AMD has
had SHA-NI for some time, newer Intel processors have it, and ARM also
has SHA-2 acceleration instructions, it's likely it will be faster on
most recent machines assuming it's compiled appropriately.
--
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA
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next prev parent reply other threads:[~2022-06-25 8:53 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
2022-06-24 15:49 ` Ævar Arnfjörð Bjarmason
2022-06-25 8:53 ` brian m. carlson [this message]
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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