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From: Junio C Hamano <>
To: Jeff King <>
Subject: Re: [PATCH 0/3] index-pack threading defaults
Date: Fri, 21 Aug 2020 11:59:58 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <> (Jeff King's message of "Fri, 21 Aug 2020 14:44:59 -0400")

Jeff King <> 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...


  reply	other threads:[~2020-08-21 19:00 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-21 17:51 [PATCH 0/3] index-pack threading defaults Jeff King
2020-08-21 17:53 ` [PATCH 1/3] p5302: disable thread-count parameter tests by default Jeff King
2020-08-21 17:54 ` [PATCH 2/3] p5302: count up to online-cpus for thread tests Jeff King
2020-08-21 17:58   ` Jeff King
2020-08-21 17:58 ` [PATCH 3/3] index-pack: adjust default threading cap Jeff King
2020-08-21 18:08   ` Eric Sunshine
2020-08-21 18:41     ` Jeff King
2020-08-22  1:16   ` brian m. carlson
2020-08-24 17:37     ` Jeff King
2020-08-24 17:55       ` Eric Sunshine
2020-08-21 18:44 ` [PATCH 0/3] index-pack threading defaults Jeff King
2020-08-21 18:59   ` Junio C Hamano [this message]
2020-08-21 19:14     ` Jeff King

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