Re: [GSoC] Git Blog 11

Christian Couder [off-list ref] 于2021年8月4日周三 下午4:57写道：

On Tue, Aug 3, 2021 at 4:48 AM ZheNing Hu [off-list ref] wrote:

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ZheNing Hu [off-list ref] 于2021年8月3日周二 上午10:37写道：

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Christian Couder [off-list ref] 于2021年8月2日周一 下午2:25写道：

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On Sun, Aug 1, 2021 at 8:45 AM ZheNing Hu [off-list ref] wrote:

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in some cases, this is the result of the performance test of
`t/perf/p1006-cat-file.sh`:

Test                                        HEAD~             HEAD
------------------------------------------------------------------------------------
1006.2: cat-file --batch-check              0.10(0.09+0.00)
0.11(0.10+0.00) +10.0%
1006.3: cat-file --batch-check with atoms   0.09(0.08+0.01)
0.09(0.06+0.03) +0.0%
1006.4: cat-file --batch                    0.62(0.58+0.04)
0.57(0.54+0.03) -8.1%
1006.5: cat-file --batch with atoms         0.63(0.60+0.02)
0.52(0.49+0.02) -17.5%

We can see that the performance of `git cat-file --batch` has been a
certain improvement!

Yeah, sure -8.1% or -17.5% is really nice! But why +10.0% for
`cat-file --batch-check`?

I think it's not very important. Because our optimization is skipping
parse_object_buffer(), git cat-file --batch-check will not set oi->contentp
by default, parse_object_buffer() will not be executed.

Do you think that if git cat-file --batch-check would set
oi->contentp, there would be no performance regression for `cat-file
--batch-check`?
Could you test that?

Oh, I mean that if git cat-file --batch-check with its default format
"%(objectname) %(objecttype)
%(objectsize)", it will not have any optimization; But if git cat-file
--batch set with "%(contents)" or
some other atoms, it will indeed be optimized. See 1006.4:

Test                                                 this tree
HEAD~
---------------------------------------------------------------------------------------------
1006.2: cat-file --batch-check                       0.15(0.12+0.02)
0.15(0.13+0.01) +0.0%
1006.3: cat-file --batch-check with basic atoms      0.12(0.10+0.01)
0.12(0.10+0.02) +0.0%
1006.4: cat-file --batch-check with contents atoms   0.66(0.63+0.02)
0.75(0.72+0.02) +13.6%
1006.5: cat-file --batch                             0.61(0.57+0.04)
0.70(0.65+0.05) +14.8%
1006.6: cat-file --batch with atoms                  0.58(0.57+0.01)
0.67(0.63+0.03) +15.5%

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Therefore, we did
not optimize `git cat-file --batch-check` at all. 10% may be small enough
for git cat-file --batch-check. The noise of environment even will cover it...

By the way, its performance may still be worse than "upstream/master", but it
will be better than before optimization.

Nice that there is some improvement, but it would be better if it was
similar to "upstream/master".

Agree.

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Test                                        HEAD~             this tree
------------------------------------------------------------------------------------
1006.2: cat-file --batch-check              0.10(0.09+0.01)
0.09(0.08+0.01) -10.0%
1006.3: cat-file --batch-check with atoms   0.09(0.07+0.02)
0.08(0.05+0.03) -11.1%
1006.4: cat-file --batch                    0.61(0.59+0.02)
0.53(0.51+0.02) -13.1%
1006.5: cat-file --batch with atoms         0.60(0.57+0.02)
0.52(0.49+0.03) -13.3%

Yeah, your patch seems to be an overall improvement when the
ref-filter code is used.

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Test                                        upstream/master   this
tree
------------------------------------------------------------------------------------
1006.2: cat-file --batch-check              0.08(0.07+0.01)
0.10(0.07+0.02) +25.0%
1006.3: cat-file --batch-check with atoms   0.06(0.05+0.01)
0.08(0.08+0.00) +33.3%
1006.4: cat-file --batch                    0.49(0.46+0.03)
0.53(0.50+0.03) +8.2%
1006.5: cat-file --batch with atoms         0.48(0.45+0.03)
0.51(0.48+0.02) +6.3%

This means that some further performance improvements are still needed
both for --batch and --batch-check though.

Have you tried to see, using gprof or something else, what is still
degrading the performance compared to when the ref-filter code isn't
used?

Yeah, gprof show that Number of calls of strbuf_add(), xstrdup() has increased
after using the logic of ref-filter. But at the same time, I noticed
that grab_person()
seems to be an area worth optimizing. grab_person() uses its parameter
"const char *who"
for type comparison, But after we added `enum atom_type` to
ref-filter, We can use it
for some comparisons. And there are two for() loops in grab_person(),
and we can merge
them into one. With this patch [1], there is a slight improvement in
performance.

Test                                                this tree
HEAD~
-------------------------------------------------------------------------------------------
1006.2: cat-file --batch-check                      0.14(0.13+0.01)
0.15(0.14+0.01) +7.1%
1006.3: cat-file --batch-check with atoms           0.12(0.10+0.01)
0.12(0.09+0.02) +0.0%
1006.4: cat-file --batch-check with contents atom   0.66(0.65+0.01)
0.66(0.64+0.02) +0.0%
1006.5: cat-file --batch                            0.60(0.57+0.02)
0.60(0.57+0.03) +0.0%
1006.6: cat-file --batch with atoms                 0.58(0.53+0.04)
0.58(0.56+0.02) +0.0%
1006.7: cat-file --batch with person atoms          0.59(0.57+0.02)
0.60(0.56+0.04) +1.7%

It’s also worth mentioning that I found that grab_person() seems to be doing
repeated parsing which parse_object_buffer() may already be done.
parse_commit_buffer()
and parse_tag_buffer() have parsed part of the content of the object,
and used by
grab_tag_values() and grab_commit_values(). For the time being, I
think this is a kind of
shallow parsing, if we can let parse_object_buffer() do in-depth
parsing, it would be great.
We can save a lot of work in grab_person()... Of course this may be a
little difficult.

Thanks.
--
ZheNing Hu

[1]: https://github.com/adlternative/git/commit/cec0ee72e64d651c01d7a2a7fe17a4adab1ef0de