Subject: Re: [PATCH v2 1/3] p5400: add perf tests for git-receive-pack(1)

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 29 Jun 2021 14:09:25 +0200

On Tue, Jun 29 2021, Patrick Steinhardt wrote:

> [[PGP Signed Part:Undecided]]
> On Mon, Jun 28, 2021 at 09:49:54AM +0200, Ævar Arnfjörð Bjarmason wrote:
>> 
>> On Mon, Jun 28 2021, Patrick Steinhardt wrote:
>> 
>> > [[PGP Signed Part:Undecided]]
>> > We'll the connectivity check logic for git-receive-pack(1) in the
>> > following commits to make it perform better. As a preparatory step, add
>> > some benchmarks such that we can measure these changes.
>> >
>> > Signed-off-by: Patrick Steinhardt <[email-masked]>
>> > ---
>> >  t/perf/p5400-receive-pack.sh | 97 ++++++++++++++++++++++++++++++++++++
>> >  1 file changed, 97 insertions(+)
>> >  create mode 100755 t/perf/p5400-receive-pack.sh
>> >
>> > diff --git a/t/perf/p5400-receive-pack.sh b/t/perf/p5400-receive-pack.sh
>> > new file mode 100755
>> > index 0000000000..a945e014a3
>> > --- /dev/null
>> > +++ b/t/perf/p5400-receive-pack.sh
>> > @@ -0,0 +1,97 @@
>> > +#!/bin/sh
>> > +
>> > +test_description="Tests performance of receive-pack"
>> > +
>> > +. ./perf-lib.sh
>> > +
>> > +test_perf_large_repo
>> 
>> From the runtime I think this just needs test_perf_default_repo, no?
>> I.e. we should only have *_large_* for cases where git.git is too small
>> to produce meaningful results.
>> 
>> Part of th problem is that git.git has become larger over time...
>
> I did these tests for 3/3 with git.git first, and results were
> significantly different. The performance issues I'm trying to fix with
> the connectivity check really only start to show up with largish
> repositories.
>
>> > +test_expect_success 'setup' '
>> > +	# Create a main branch such that we do not have to rely on any specific
>> > +	# branch to exist in the perf repository.
>> > +	git switch --force-create main &&
>> > +
>> > +	# Set up a pre-receive hook such that no refs will ever be changed.
>> > +	# This easily allows multiple perf runs, but still exercises
>> > +	# server-side reference negotiation and checking for consistency.
>> > +	mkdir hooks &&
>> > +	write_script hooks/pre-receive <<-EOF &&
>> > +		#!/bin/sh
>> 
>> You don't need the #!/bin/sh here, and it won't be used. write_script()
>> adds it (or the wanted shell path).
>
> Makes sense.
>
>> > +		echo "failed in pre-receive hook"
>> > +		exit 1
>> > +	EOF
>> > +	cat >config <<-EOF &&
>> > +		[core]
>> > +			hooksPath=$(pwd)/hooks
>> > +	EOF
>> 
>> Easier understood IMO as:
>> 
>>     git config -f config core.hooksPath ...
>
> Yup, will change.
>
>> > +	GIT_CONFIG_GLOBAL="$(pwd)/config" &&
>> > +	export GIT_CONFIG_GLOBAL &&
>> > +
>> > +	git switch --create updated &&
>> > +	test_commit --no-tag updated
>> > +'
>> > +
>> > +setup_empty() {
>> > +	git init --bare "$2"
>> > +}
>> 
>> I searched ahead for setup_empty, looked unused, but...
>> 
>> > +setup_clone() {
>> > +	git clone --bare --no-local --branch main "$1" "$2"
>> > +}
>> > +
>> > +setup_clone_bitmap() {
>> > +	git clone --bare --no-local --branch main "$1" "$2" &&
>> > +	git -C "$2" repack -Adb
>> > +}
>> > +
>> > +# Create a reference for each commit in the target repository with extra-refs.
>> > +# While this may be an atypical setup, biggish repositories easily end up with
>> > +# hundreds of thousands of refs, and this is a good enough approximation.
>> > +setup_extrarefs() {
>> > +	git clone --bare --no-local --branch main "$1" "$2" &&
>> > +	git -C "$2" log --all --format="tformat:create refs/commit/%h %H" |
>> > +		git -C "$2" update-ref --stdin
>> > +}
>> > +
>> > +# Create a reference for each commit in the target repository with extra-refs.
>> > +# While this may be an atypical setup, biggish repositories easily end up with
>> > +# hundreds of thousands of refs, and this is a good enough approximation.
>> > +setup_extrarefs_bitmap() {
>> > +	git clone --bare --no-local --branch main "$1" "$2" &&
>> > +	git -C "$2" log --all --format="tformat:create refs/commit/%h %H" |
>> > +		git -C "$2" update-ref --stdin &&
>> > +	git -C "$2" repack -Adb
>> > +}
>> > +
>> > +for repo in empty clone clone_bitmap extrarefs extrarefs_bitmap
>> > +do
>> > +	test_expect_success "$repo setup" '
>> 
>> > +		rm -rf target.git &&
>> > +		setup_$repo "$(pwd)" target.git
>> 
>> ...here we use it via interpolation.
>> 
>> I'd find this whole pattern much easier to understand if the setups were
>> just a bunch of test_expect_success that created a repo_empty.git,
>> repo_extrarefs.git etc. Then this loop would be:
>> 
>>     for repo in repo*.git ...
>> 
>> I'd think that would also give you more meaningful perf data, as now the
>> OS will churn between the clone & the subsequent push tests, better to
>> do all the setup, then all the different perf tests.
>> 
>> Perhaps there's also a way to re-use this setup across different runs, I
>> don't know/can't remember if t/perf has a less transient thing than the
>> normal trash directory to use for that.
>
> I originally had code like this, but the issue with first creating all
> the repos is that it requires lots of disk space with large repos given
> that we'll clone it once per setup. Combined with the fact that I
> often run tests in tmpfs, this led to out-of-memory situations quite
> fast given that I had 3x6GB repositories plus the seeded packfiles in
> RAM.
>
> This is why I've changed the setup to do the setup as we go, to bring
> disk usage down to something sane.
>
> Patrick
>
> [[End of PGP Signed Part]]

Ah, I see. In that case wouldn't it be even better/faster with/without
my suggestion to not use "clone" here, which would either be manually
set up with alternates, or removing the --no-local flag.

You'd then share bulk of the object database, and just have different
references. B.t.w. you'll probably get less noise/more relevant results
if you then do a "pack-refs" after creating those N references.

So you should have:

 0. Your "big" test repop (not used directly)
 1. An empty repo
 2. The "big" test repo itself, but just the HEAD branch, using
    alternates to point to #0
 3. Ditto, but we create a crapload of refs for each commit for a
    version of #2.
 4. Ditto #3 (could even "cp" over the packed refs file to save time),
    but add a bitmap on top.

Well, presumably for #4 we'd actually want to do the "git repack -Adb"
for #2 (or enforce that #0 must have it), then just move the *.bitmap
file(s) to #4. Now the test case conflates whether we have bitmaps with
how well (re)packed something is.

I think this might also allow you to get rid of the pre-receive hook for
a "real" push test, since the side-repos would be so cheap at this point
that you could perhaps setup N of them to push into.