Re: time needed to rebase shortend by using --onto?
From: Elijah Newren <hidden>
Date: 2021-05-27 23:08:46
On Thu, May 27, 2021 at 2:59 PM Uwe Kleine-König [off-list ref] wrote:
Hello, On Wed, May 26, 2021 at 07:38:08AM -0700, Elijah Newren wrote:quoted
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
Note: In your original report you had rename detection and it clearly took a significant amount of time...
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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)
And here, there was no rename detection so this isn't the same thing anymore. You could try setting merge.renameLimit higher. However, the 7-8 second difference (and the likely large differences between 5.4 and 5.10) do suggest that Junio's hunch that fork-point behavior being at play could be an issue in these two commands.
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.
Yeah, with no rename detection, the newer git version isn't going to make a bit of difference.
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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
So, not freshly packed, but not in need of an automatic gc either.
alternate: /home/uwe/var/gitstore/linux.git/objects
You've got an alternate? How well packed is it? (What does "git count-objects -v" in that other repo show?)
(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?)If there isn't some still-running git operation that is fetching and writing to these files, then yes they can be cleaned out. I doubt they'd make too much of a difference, though. I was more curious if you went from say 10000 loose objects to ~0, or from 50+ packs down to 1 between operations due to an automatic gc completing.
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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.
Right, this says that --strategy=ort WITH rename detection is as fast as the default --strategy=recursive WITHOUT rename detection. It's not a fair comparison (you'd need to set merge.renameLimit higher and re-run the cases where you had warnings), but is interesting nonetheless. It basically suggests that rename detection comes for free with the ort strategy.
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.
Thanks.