Re: time needed to rebase shortend by using --onto?

From: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Date: May 27, 2021, 21:59 UTC
Message-ID: <20210527215947.g2mnds6zj5uv5mjq@pengutronix.de>
In-Reply-To: <CABPp-BGBY9kwqRQ+soa8=W2F+=8eQRYS3vWS_7UCC0K0qNTW1g@mail.gmail.com>

Hello,

On Wed, May 26, 2021 at 07:38:08AM -0700, Elijah Newren wrote:

Show 58 quoted lines

> On Wed, May 26, 2021 at 3:13 AM Uwe Kleine-König
> <u.kleine-koenig@pengutronix.de> wrote:
> > I have a kernel topic branch containing 4 patches on top of Linux v5.4.
> > (I didn't speak to the affected customer, so I cannot easily share the
> > patch stack. If need be I can probably anonymize it or ask if I can
> > publish the patches.)
> >
> > It rebases clean on v5.10:
> >
> >         $ time git rebase v5.10
> >         Performing inexact rename detection: 100% (36806539/36806539), done.
> >         Performing inexact rename detection: 100% (36806539/36806539), done.
> >         Performing inexact rename detection: 100% (36806539/36806539), done.
> >         Performing inexact rename detection: 100% (36806539/36806539), done.
> >         Successfully rebased and updated detached HEAD.
> >
> >         real    3m47.841s
> >         user    1m25.706s
> >         sys     0m11.181s
> >
> > If I start with the same rev checked out and explicitly specify the
> > merge base, the rebase process is considerably faster:
> >
> >         $ time git rebase --onto v5.10 v5.4
> >         Performing inexact rename detection: 100% (36806539/36806539), done.
> >         Performing inexact rename detection: 100% (36806539/36806539), done.
> >         Performing inexact rename detection: 100% (36806539/36806539), done.
> >         Performing inexact rename detection: 100% (36806539/36806539), done.
> >         Successfully rebased and updated detached HEAD.
> >
> >         real    1m20.588s
> >         user    1m12.645s
> >         sys     0m6.733s
> >
> > Is there some relevant complexity in the first invocation I'm not seeing
> > that explains it takes more than the double time? I would have expected
> > that
> >
> >         git rebase v5.10
> >
> > does the same as:
> >
> >         git rebase --onto v5.10 $(git merge-base HEAD v5.10)
> >
> > . (FTR:
> >
> >         $ time git merge-base HEAD v5.10
> >         219d54332a09e8d8741c1e1982f5eae56099de85
> >
> >         real    0m0.158s
> >         user    0m0.105s
> >         sys     0m0.052s
> >
> > , 219d5433 is v5.4 as expected.
> 
> That does seem surprising, though if an automatic gc completed between
> the two commands that could certainly explain it.  If that theory is
> correct, it would suggest that it'd be difficult for you to reproduce;

This reproduces just fine. The repository is quite big and it is slow at times. With the same tree on a different machine, the rebase is quicker, but the factor 2 between the two different commands is visible there, too:

uwe@taurus:~/gsrc/linux$ git checkout bc2e99c9c9e0d29494b1739624554e4f5f979d32 HEAD is now at bc2e99c9c9e0 [...]

uwe@taurus:~/gsrc/linux$ time git rebase v5.10 warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8604 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8604 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8604 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8604 and retry the command. Successfully rebased and updated detached HEAD.

real 0m20.737s user 0m14.188s sys 0m3.767s

uwe@taurus:~/gsrc/linux$ git checkout bc2e99c9c9e0d29494b1739624554e4f5f979d32 HEAD is now at bc2e99c9c9e0 [...]

uwe@taurus:~/gsrc/linux$ time git rebase --onto v5.10 v5.4 warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8604 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8604 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8604 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8604 and retry the command. Successfully rebased and updated detached HEAD.

real 0m12.129s user 0m7.196s sys 0m3.141s

(This is with a slightly newer git: 2.30.2-1 from Debian)

Then I repeated the test with git 2.32.0-rc1 (wgit is just calling bin-wrappers/git in my git working copy):

uwe@taurus:~/gsrc/linux$ wgit version git version 2.32.0.rc1

uwe@taurus:~/gsrc/linux$ wgit checkout bc2e99c9c9e0d29494b1739624554e4f5f979d32 HEAD is now at bc2e99c9c9e0 [...]

uwe@taurus:~/gsrc/linux$ time wgit rebase v5.10 warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8024 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8024 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8024 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8024 and retry the command. Successfully rebased and updated detached HEAD.

real 0m19.438s user 0m13.629s sys 0m3.299s

uwe@taurus:~/gsrc/linux$ wgit checkout bc2e99c9c9e0d29494b1739624554e4f5f979d32 HEAD is now at bc2e99c9c9e0 [...]

uwe@taurus:~/gsrc/linux$ time wgit rebase --onto v5.10 v5.4 warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8024 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8024 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8024 and retry the command. warning: inexact rename detection was skipped due to too many files. warning: you may want to set your merge.renamelimit variable to at least 8024 and retry the command. Successfully rebased and updated detached HEAD.

real 0m13.848s user 0m8.315s sys 0m3.182s

So the surprise persists.

> running again with either command would give you something closer to
> the lower time both times.  Is that the case?  (Also, what's the
> output of "git count-objects -v"?)

After the above commands I have:

	count: 3203
	size: 17664
	in-pack: 4763753
	packs: 11
	size-pack: 1273957
	prune-packable: 19
	garbage: 0
	size-garbage: 0
	alternate: /home/uwe/var/gitstore/linux.git/objects

(On the repository I did this initially I have:

	warning: garbage found: .git/objects/pack/pack-864148a84c0524073ed8c8aa1a76155d5c677879.pack.temp
	warning: garbage found: /ptx/src/git/linux.git/objects/pack/tmp_pack_X9gHnq
	count: 2652
	size: 14640
	in-pack: 2117015
	packs: 8
	size-pack: 574167
	prune-packable: 856
	garbage: 2
	size-garbage: 1114236
	alternate: /ptx/src/git/linux.git/objects

(Is the garbage a reason this is so slow? Can I just remove the two files pointed out?)

Show 6 quoted lines

> I'd love to try this with git-2.32.0-rc1 (or even my not-yet-upstream
> patches that optimize even further) with adding "--strategy=ort" to
> your rebase command to see how much of a timing difference it makes.
> Any chance the patches could either be published, or you could retry
> with git-2.32.0-rc1 and add the --strategy=ort command line option to
> your rebase command(s)?

With --strategy=ort added I have:

uwe@taurus:~/gsrc/linux$ time wgit rebase --strategy=ort v5.10 Successfully rebased and updated detached HEAD.

real 0m19.202s user 0m12.724s sys 0m2.961s

[...]

uwe@taurus:~/gsrc/linux$ time wgit rebase --strategy=ort --onto v5.10 v5.4 Successfully rebased and updated detached HEAD.

real 0m12.395s user 0m6.638s sys 0m3.284s

So the warnings about inexact rename detection don't appear and it's a bit faster, but I still see the timing difference between these two commands.

I assume you are still interested in seeing this branch? I think anonymising it shouldn't be so hard, the patches are not so big. I'll modify the branch to make it shareable and assuming the problem still reproduces with it will share it with you.

Best regards Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |