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

[Jeff King <peff@peff.net>]

On Fri, Aug 21, 2020 at 11:59:58AM -0700, Junio C Hamano wrote:

> > 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...

Yeah, I agree on the "real world" part, but I'm the curious sort, so
here are numbers compiled against openssl (which is generally even
faster than block-sha1, and would thus emphasize the results of our
hypothesis):

  5302.3: index-pack 0 threads                   108.78(106.39+2.31)
  5302.4: index-pack 1 threads                   110.65(108.08+2.49)
  5302.5: index-pack 2 threads                   67.57(110.83+2.75) 
  5302.6: index-pack 4 threads                   48.18(123.82+3.02) 
  5302.7: index-pack 8 threads                   39.07(153.45+4.13) 
  5302.8: index-pack 16 threads                  38.38(265.78+7.71) 
  5302.9: index-pack default number of threads   54.64(117.35+2.73)

So it's actually pretty similar. Things continue getting faster as we go
past 3 threads. Though our total improvement is less (29% better with 8
threads compared to 3, versus 42% better when using sha1dc). So I think
it's _part_ of the reason, but not all of it.

-Peff