Re: Unreliable 'git rebase --onto'

[Elijah Newren <newren@gmail.com>]

Hi,

On Thu, Jan 9, 2020 at 7:03 AM SZEDER Gábor <szeder.dev@gmail.com> wrote:
>
> On Wed, Jan 08, 2020 at 04:55:46PM -0800, Elijah Newren wrote:
> > > Alas, there is unexpected bad news: with that commit the runtime of
> > > your 'git rebase --onto' command goes from <1sec to over 50secs.
> > > Cc-ing Elijah, author of that patch...
> >
> > I see slowdown, but not nearly as big as you report:
>
> The linux repo is big, my notebook is small, the poor thing :)

It went to just over 64secs on my home laptop (older and with spinny
disks), so yeah, a big difference from my work machine which has an
SSD.

> > $ time git rebase -m --onto v4.18 463fa44eec2fef50~ 463fa44eec2fef50
> > warning: inexact rename detection was skipped due to too many files.
> > warning: you may want to set your merge.renamelimit variable to at
> > least 7216 and retry the command.
> > Successfully rebased and updated detached HEAD.
> >
> > real 0m13.305s
> > user 0m9.644s
> > sys 0m3.620s
>
> > Interestingly, turning off rename detection only speeds it up a little bit:
> > $ time git rebase -m -Xno-renames --onto v4.18 463fa44eec2fef50~
> > 463fa44eec2fef50
> > Successfully rebased and updated detached HEAD.
> >
> > real 0m11.955s
> > user 0m8.732s
> > sys 0m3.424s
> >
> >
> > This is an interesting testcase; I'm going to try to find some time to
> > dig in further.
>
> The culprits are two seemingly unnecessary back-and-forth checkouts.
>
> I didn't realize I could use 'git rebase -m', so ran some tests with
> it, and turns out that the slowdown started with 68aa495b59 (rebase:
> implement --merge via the interactive machinery, 2018-12-11), where
> the runtime suddenly went from <1.5s to 45+s.
>
> Running 'git rebase -i --onto <those-same-commits>' is just as slow,
> and it appears that it has always been (the oldest I tried was
> v1.8.0), and it spends a long time both before and after popping up
> the editor for the rebase instructions.  That's highly suspicious, so:
>
>   $ git log --oneline -1
>   94710cac0ef4 (HEAD, tag: v4.18) Linux 4.18
>   $ git rebase -i --onto v4.18 463fa44eec2fef50~ 463fa44eec2fef50
>   hint: Waiting for your editor to close the file...
>   # Hit ctrl-z in the editor
>   $ git log --oneline -1
>   463fa44eec2f (HEAD) Input: atmel_mxt_ts - disable IRQ across suspend
>
> Oh.
>
> So 'git rebase -i' apparently checks out the tip commit of the
> to-be-rebased revision range before invoking the editor for the rebase
> instructions, only to check out the --onto commit (i.e. the commit
> we've started from!) to apply the selected commit on top.
>
> And indeed those two checkouts account for all the wasted runtime:
>
>   $ time { git checkout 463fa44eec2fef50 && git checkout v4.18 ; }
>   Updating files: 100% (49483/49483), done.
>   Previous HEAD position was 94710cac0ef4 Linux 4.18
>   HEAD is now at 463fa44eec2f Input: atmel_mxt_ts - disable IRQ across suspend
>   Updating files: 100% (49483/49483), done.
>   Previous HEAD position was 463fa44eec2f Input: atmel_mxt_ts - disable IRQ across suspend
>   HEAD is now at 94710cac0ef4 Linux 4.18
>
>   real    0m48.801s
>   user    0m13.963s
>   sys     0m5.114s

Oh, cool, sounds like you're already investigating and found the problem.