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

[Eric Sunshine <sunshine@sunshineco.com>]

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.