Re: [PATCH 1/1] commit-graph.c: avoid unnecessary tag dereference when merging

[Taylor Blau <me@ttaylorr.com>]

On Sat, Mar 21, 2020 at 12:11:41AM -0600, Taylor Blau wrote:
> On Sat, Mar 21, 2020 at 01:00:25AM -0400, Jeff King wrote:
> > On Fri, Mar 20, 2020 at 09:44:23PM -0600, Taylor Blau wrote:
> >
> > > In my testing environment, this improves the time to "merge" a split
> > > commit-graph containing all reachable commits in the kernel by
> > > re-writing the same commit-graph (effectively measuring the time it
> > > takes to check that all of those commits still exist) from:
> > >
> > >   Attempt 1: 9.614
> > >   Attempt 2: 10.984
> > >   Attempt 3: 10.39
> > >   Attempt 4: 9.14
> > >   Attempt 5: 9.439
> > >
> > >   real	0m9.140s
> > >   user	0m8.207s
> > >   sys	0m0.602s
> > >
> > > to:
> > >
> > >   Attempt 1: 9.12
> > >   Attempt 2: 8.904
> > >   Attempt 3: 9.361
> > >   Attempt 4: 9.288
> > >   Attempt 5: 9.677
> > >
> > >   real	0m8.904s
> > >   user	0m8.208s
> > >   sys	0m0.596s
> > >
> > > yielding a modest ~2.6% improvement in the best timings from each run,
> > > and ~7.4% improvement on average.
> >
> > That still seems really slow to me. If we were truly eliding the load of
> > most of the commit objects, I'd expect an order of magnitude or so
> > improvement. For example, with a fully constructed commit graph in
> > linux.git, I get:
> >
> >   $ time git -c core.commitGraph=1 rev-list HEAD | wc -l
> >   886922
> >
> >   real	0m1.088s
> >   user	0m0.659s
> >   sys	0m1.161s
> >
> >   $ time git -c core.commitGraph=0 rev-list HEAD | wc -l
> >   886922
> >
> >   real	0m7.185s
> >   user	0m6.729s
> >   sys	0m1.882s
> >
> > Obviously not the same operation, but that should give us a rough idea
> > that commit graph lookups are 6-7 times cheaper than loading the actual
> > objects. I don't remember the details of the case that originally led us
> > towards this patch. Can you share more of the setup you used to generate
> > the numbers above (which repo, but more importantly the commands to
> > create the initial state and then time the test).
>
> Sure. I'm running best-of-five on the time it takes to re-generate and
> merge a commit-graph based on in-pack commits.
>
> The script is (in linux.git):
>
>   $ best-of-five \
>       -p 'rm -rf .git/objects/info/commit-graph{,s/}; git commit-graph write --split=no-merge 2>/dev/null' \
>       git commit-graph write --split=merge-all
>
> So we're measuring the time it takes to crawl all the packs, decide on
> the splitting strategy, and then compare all commits in the new merged
> graph to make sure that they don't already exist in the object store.
>
> But, here's where things get... Bizarre. I was trying to come up with a
> way to do fewer things and spend proportionally more time in
> 'merge_commit_graphs', so I did something like:
>
>   - Generate a pack containing a single, empty commit.
>   - Generate a split commit-graph containing commits in the single large
>     pack containing all of history.
>   - Generate a commit-graph of the small pack, and merge it with the
>     large pack.
>
> That script is:
>
>   $ git --version
>   $ git commit -m "empty" --allow-empty
>   $ pack="pack-$(git rev-parse HEAD | git pack-objects .git/objects/pack/pack).idx"
>   $ best-of-five \
>       -p "rm -rf .git/objects/info/commit-graphs && cp -r .git/objects/info/commit-graphs{.bak,}" \
>       sh -c "echo $pack | git commit-graph write --split=merge-all"
>
> but things get... slower with this patch? Here are the results before
> and after:
>
>   Attempt 1: 8.444
>   Attempt 2: 8.453
>   Attempt 3: 8.391
>   Attempt 4: 8.376
>   Attempt 5: 7.859
>
>   real	0m7.859s
>   user	0m7.309s
>   sys	0m0.511s
>
> vs:
>
>   Attempt 1: 8.69
>   Attempt 2: 8.735
>   Attempt 3: 8.619
>   Attempt 4: 8.747
>   Attempt 5: 8.695
>
>   real	0m8.619s
>   user	0m8.030s
>   sys	0m0.538s
>
> Without more profiling, I'm puzzled by why this patch seems to make
> things *worse* under this scenario.

Hmm. I tried roughly the same setup as the latter of the above two, but
this time under perf. On a warm cache, it does look like this parch is
an improvement:

  $ sudo perf diff perf.data perf.data.patch | grep merge_commit
       0.11%   -0.02%  git                [.] merge_commit_graph

(perf.data is from running 'next', perf.data.patch is from running
'next' with this patch applied on top). So... a slight improvement, but
nothing like what I had reported in the original patch.

I am puzzled.

> So, I'd appreciate your thoughts: does this set-up seem reasonable? Is
> it introducing some latency that isn't being accounted for in the
> original setup?
>
> > The patch otherwise still makes sense to me, but I suspect there are
> > other similar optimizations nearby that we'll need to do in tandem.
> >
> > -Peff
>
> Thanks,
> Taylor

Thanks,
Taylor