Re: [PATCH 3/3] index-pack: adjust default threading cap

From: Eric Sunshine <sunshine@sunshineco.com>
To: Jeff King <peff@peff.net>
Cc: Git List <git@vger.kernel.org>
Subject: Re: [PATCH 3/3] index-pack: adjust default threading cap
Date: Fri, 21 Aug 2020 14:08:55 -0400	[thread overview]
Message-ID: <CAPig+cRQG6EN7Zq_fYMQOM7y9a6rgwWORZhN=px21-7RorWNdg@mail.gmail.com> (raw)
In-Reply-To: <20200821175800.GC3263141@coredump.intra.peff.net>

On Fri, Aug 21, 2020 at 1:58 PM Jeff King <peff@peff.net> wrote:
> So what's a good default value? It's clear that the current cap of 3 is
> too low; our default values are 42% and 57% slower than the best times
> on each machine. The results on the 40-core machine imply that 20
> threads is an actual barrier regardless of the number of cores, so we'll
> take that as a maximum. We get the best results on these machines at
> half of the online-cpus value. That's presumably a result of the
> hyperthreading. That's common on multi-core Intel processors, but not
> necessarily elsewhere. But if we take it as an assumption, we can
> perform optimally on hyperthreaded machines and still do much better
> than the status quo on other machines, as long as we never half below
> the current value of 3.

I'm not familiar with the index-pack machinery, so this response may
be silly, but the first question which came to my mind was whether or
not SSD vs. spinning-platter disk impacts these results, and which of
the two you were using for the tests (which I don't think was
mentioned in any of the commit messages). So, basically, I'm wondering
about the implication of this change for those of us still stuck with
old spinning-platter disks.