Re: [PATCH 0/3] index-pack threading defaults

From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: git@vger.kernel.org
Subject: Re: [PATCH 0/3] index-pack threading defaults
Date: Fri, 21 Aug 2020 11:59:58 -0700	[thread overview]
Message-ID: <xmqqmu2n964x.fsf@gitster.c.googlers.com> (raw)
In-Reply-To: <20200821184459.GB3263614@coredump.intra.peff.net> (Jeff King's message of "Fri, 21 Aug 2020 14:44:59 -0400")

Jeff King <peff@peff.net> writes:

> On Fri, Aug 21, 2020 at 01:51:53PM -0400, Jeff King wrote:
>
>> That value was determined experimentally in 2012. I'm not sure of the
>> exact reason it's different now (modern processors are better at
>> parallelism, or modern git is better at parallelism, or the original
>> experiment was just a fluke). But regardless, I can get on the order of
>> a two-to-one speedup by bumping the thread count. See the final patch
>> for timings and more specific discussion.
>
> After writing a response elsewhere in the thread, it occurred to me that
> a good candidate for explaining this may be that our modern sha1dc
> implementation is way slower than what we were using in 2012 (which
> would have been either block-sha1, or the even-faster openssl
> implementation). And since a good chunk of index-pack's time is going to
> computing sha1 hashes on the resulting objects, that means that since
> 2012, we're spending relatively more time in the hash computation (which
> parallelizes per-object) and less time in the other parts that happen
> under a lock.

Believable conjecture that is.  You could benchmark again with
block-sha1 on today's hardware, but because the performance profile
with sha1dc is what matters in the real world anyway...

Thanks.