Re: [PATCH v3 16/16] midx.c: improve cache locality in midx_pack_order_cmp()

[Taylor Blau <me@ttaylorr.com>]

On Mon, Mar 29, 2021 at 08:59:12AM -0400, Jeff King wrote:
> On Thu, Mar 11, 2021 at 12:05:42PM -0500, Taylor Blau wrote:
>
> > From: Jeff King <peff@peff.net>
> >
> > There is a lot of pointer dereferencing in the pre-image version of
> > 'midx_pack_order_cmp()', which this patch gets rid of.
> >
> > Instead of comparing the pack preferred-ness and then the pack id, both
> > of these checks are done at the same time by using the high-order bit of
> > the pack id to represent whether it's preferred. Then the pack id and
> > offset are compared as usual.
> >
> > This produces the same result so long as there are less than 2^31 packs,
> > which seems like a likely assumption to make in practice.
>
> Obviously this patch is brilliant. ;)

Obviously.

> Did we record any numbers to show the improvement here? I don't think it
> can be demonstrated with this series (since most of the code is dead),
> but I recall that this was motivated by a noticeable slowdown.

Looking through our messages, you wrote that this seemed to produce a
.8 second speed-up on a large-ish repository that we were testing.
That's not significant overall, the fact that we were spending so long
probably caught our attention when looking at a profiler.

I could go either way on mentioning it. It does feel a little like
cheating to say, "well, if you applied these other patches it would make
it about this much faster". So I'm mostly happy to just keep it vague
and say that it makes things a little faster, unless you feel strongly
otherwise.

> I briefly wondered whether the complicated midx_pack_order_cmp() in
> pack-revindex.c, which is used for the bsearch() there, could benefit
> from the same speedup. It's only log(n), of course, instead of n*log(n),
> but one might imagine making "n" calls to it. I don't think it makes
> sense, though. The pointer dereferencing there is into the midx mmap
> itself. Creating an auxiliary array would defeat the purpose.

Right.

> -Peff

Thanks,
Taylor