Re: [PATCH v2 6/7] merge-ort: avoid recursing into directories when we don't need to
- From
Elijah Newren <newren@gmail.com>
- Date
- Jul 15, 2021, 16:28 UTC
- Message-ID
- <CABPp-BHk6=WR-T-tFJ8v_xpqp0emW8Z29VkCkxePfR_tuUcBJw@mail.gmail.com>
- In-Reply-To
- <24c5ef26-f5d3-fb0e-b2da-24cdac90450d@gmail.com>
On Thu, Jul 15, 2021 at 7:55 AM Derrick Stolee <stolee@gmail.com> wrote:
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>
> On 7/13/2021 3:33 PM, Elijah Newren via GitGitGadget wrote:
> > From: Elijah Newren <newren@gmail.com>
> ...
> > For the testcases mentioned in commit 557ac0350d ("merge-ort: begin
> > performance work; instrument with trace2_region_* calls", 2020-10-28),
> > this change improves the performance as follows:
> >
> > Before After
> > no-renames: 5.235 s ± 0.042 s 205.1 ms ± 3.8 ms
>
> Wow! This is quite the savings, and reinforces that when no renames
> exist we are likely to hit this optimization.Just to clarify, "no-renames" was an unfortunate misnomer. I should have called it "few-renames".
But yeah, I was pretty amazed a year ago (when I initially implemented this optimization) seeing the speedups on this testcase -- a testcase that I hadn't even been paying much attention to beyond trying to avoid regressing it.
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> > mega-renames: 9.419 s ± 0.107 s 1.564 s ± 0.010 s > > I'm surprised that this one works so well, too. Clearly, there must > be a lot of directories that can be skipped despite many renames > existing.
One small reason it works is that some of the commits don't touch any paths involved in the big directory rename. That means that for those commits, all the renames under that big directory are irrelevant. This fact makes the optimization work for 4 of the 35 patches. (Of course, I did pick that particular series of commits for rebasing because it touched the directory being renamed a lot, including adding files to it, which made it a better stress test case for my optimizations. But that does make it less likely for the irrelevant-renames criteria to help us out on this particular optimization.)
The bigger reason this optimization works so well on this testcase, is the mere fact that this rebase involved a sequence of 35 commits. The optimization doesn't work for the first commit (as noted by the just-one-mega testcase below). However, once the first commit is picked, renames on the upstream side that were relevant in the first commit are cached, so the optimization does trigger for many of the subsequent commits. And whenever the optimization doesn't work on a subsequent commit -- which happens because the new commit modifies different files that make other renames become relevant -- then after detecting the new renames those also get cached. More cached renames means a higher likelihood that we can skip recursing into directories when rebasing subsequent commits.
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> > just-one-mega: 480.1 ms ± 3.9 ms 479.5 ms ± 3.9 ms
>
> And no overhead added to this case. Good.
>
> > + /* Loop over the set of paths we need to know rename info for */
> > + strset_for_each_entry(&renames->relevant_sources[side],
> > + &iter, entry) {
> > + char *rename_target, *dir, *dir_marker;
> > + struct strmap_entry *e;
> > +
> > + /*
> > + * if we don't know delete/rename info for this path,
> > + * then we need to recurse into all trees to get all
> > + * adds to make sure we have it.
> > + */
>
> super-nit: s/if/If/Will fix.
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> Otherwise, I can't speak to the code being 100% correct because > this area is so dense. I find the code to be well organized, > which will help finding and fixing any potential bugs that might > show up in strange corner cases later. > > Thanks, > -Stolee