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

[Jeff King <peff@peff.net>]

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.

-Peff