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