Hello,

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

On Wed, May 26, 2021 at 3:13 AM Uwe Kleine-König
[off-list ref] wrote:

quoted

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?)

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/ |