Re: [PATCH v2 2/3] receive-pack: skip connectivity checks on delete-only commands

From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Patrick Steinhardt <ps@pks.im>
Cc: git@vger.kernel.org, "Jeff King" <peff@peff.net>,
	"Felipe Contreras" <felipe.contreras@gmail.com>,
	"SZEDER Gábor" <szeder.dev@gmail.com>,
	"Chris Torek" <chris.torek@gmail.com>,
	"Junio C Hamano" <gitster@pobox.com>
Subject: Re: [PATCH v2 2/3] receive-pack: skip connectivity checks on delete-only commands
Date: Mon, 28 Jun 2021 10:06:09 +0200	[thread overview]
Message-ID: <87v95ywgx8.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <87y2auwh1d.fsf@evledraar.gmail.com>

On Mon, Jun 28 2021, Ævar Arnfjörð Bjarmason wrote:

> On Mon, Jun 28 2021, Patrick Steinhardt wrote:
>
>> [[PGP Signed Part:Undecided]]
>> In the case where git-receive-pack(1) receives only commands which
>> delete references, then per technical specification the client MUST NOT
>> send a packfile. As a result, we know that no new objects have been
>> received, which makes it a moot point to check whether all received
>> objects are fully connected.
>
> Is it just per specification, or do we also have assertions/tests for
> what happens in that case?
>
>> [...]
>> The following tests were executed on linux.git and back up above
>> expectation:
>>
>> Test                                                  origin/master           HEAD
>> ---------------------------------------------------------------------------------------------------------
>> 5400.4: empty receive-pack updated:new                178.36(428.22+164.36)   177.62(421.33+164.48) -0.4%
>> 5400.7: clone receive-pack updated:new                0.10(0.08+0.02)         0.10(0.08+0.02) +0.0%
>> 5400.9: clone receive-pack updated:main               0.10(0.08+0.02)         0.11(0.08+0.02) +10.0%
>> 5400.11: clone receive-pack main~10:main              0.15(0.11+0.04)         0.15(0.10+0.05) +0.0%
>> 5400.13: clone receive-pack :main                     0.01(0.00+0.01)         0.01(0.01+0.00) +0.0%
>> 5400.16: clone_bitmap receive-pack updated:new        0.10(0.07+0.02)         0.09(0.06+0.02) -10.0%
>> 5400.18: clone_bitmap receive-pack updated:main       0.10(0.07+0.02)         0.10(0.08+0.02) +0.0%
>> 5400.20: clone_bitmap receive-pack main~10:main       0.15(0.11+0.03)         0.15(0.12+0.03) +0.0%
>> 5400.22: clone_bitmap receive-pack :main              0.02(0.01+0.01)         0.01(0.00+0.00) -50.0%
>> 5400.25: extrarefs receive-pack updated:new           32.34(20.72+11.86)      32.56(20.82+11.95) +0.7%
>> 5400.27: extrarefs receive-pack updated:main          32.42(21.02+11.61)      32.52(20.64+12.10) +0.3%
>> 5400.29: extrarefs receive-pack main~10:main          32.53(20.74+12.01)      32.39(20.63+11.97) -0.4%
>> 5400.31: extrarefs receive-pack :main                 7.13(3.53+3.59)         7.15(3.80+3.34) +0.3%
>> 5400.34: extrarefs_bitmap receive-pack updated:new    32.55(20.72+12.04)      32.65(20.68+12.18) +0.3%
>> 5400.36: extrarefs_bitmap receive-pack updated:main   32.50(20.90+11.86)      32.67(20.93+11.94) +0.5%
>> 5400.38: extrarefs_bitmap receive-pack main~10:main   32.43(20.88+11.75)      32.35(20.68+11.89) -0.2%
>> 5400.40: extrarefs_bitmap receive-pack :main          7.21(3.58+3.63)         7.18(3.61+3.57) -0.4%
>
> We're doing less work so I'd expect to te be faster, but do these tests
> really back that up? From eyeballing these I can't find a line where the
> confidence intervals don't overlap, e.g. the +10% regresison is a
> .10->.11 "regression" with a [+-] 0.02 (so within the error bars) etc,
> ditto for the -50% improvement.
>
> Perhaps the error bars will reduce with a high GIT_PERF_REPEAT_COUNT, or
> the re-arrangement for keeping things hotter in cache that I suggested
> in 1/3.

Urgh, nevermind. Just after I sent this I re-read t/perf/README. I'd
somehow recalled that we'd emit error bars from it, but that's
Elapsed(User + System), not Time(.. + errorbar) as I thought, nevermind.
Still, numbers are such that I wonder if the differences are getting
lost in some noise...