Git Mailing List Archive on lore.kernel.org
 help / color / Atom feed
From: "SZEDER Gábor" <szeder.dev@gmail.com>
To: Elijah Newren <newren@gmail.com>
Cc: "Eugeniu Rosca" <erosca@de.adit-jv.com>,
	"Junio C Hamano" <gitster@pobox.com>, "Jeff King" <peff@peff.net>,
	"Ævar Arnfjörð" <avarab@gmail.com>,
	"Git Mailing List" <git@vger.kernel.org>,
	"Eugeniu Rosca" <roscaeugeniu@gmail.com>
Subject: Re: Unreliable 'git rebase --onto'
Date: Thu, 9 Jan 2020 16:03:32 +0100
Message-ID: <20200109150332.GF32750@szeder.dev> (raw)
In-Reply-To: <CABPp-BHsy75UGm4wTOP2_AYik_dZi-_BxtAn-hyi-ZrNRRWGuw@mail.gmail.com>

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 :)

> $ 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


  reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-08 21:43 Eugeniu Rosca
2020-01-08 22:35 ` SZEDER Gábor
2020-01-09  0:55   ` Elijah Newren
2020-01-09 15:03     ` SZEDER Gábor [this message]
2020-01-09 17:53       ` Elijah Newren
2020-01-21 19:18       ` [PATCH v1] rebase -i: stop checking out the tip of the branch to rebase Alban Gruin
2020-01-21 20:07         ` Elijah Newren
2020-01-22 20:24         ` Junio C Hamano
2020-01-22 20:47         ` Junio C Hamano
2020-01-24 14:45           ` Alban Gruin
2020-01-24 14:45         ` [PATCH v2] " Alban Gruin
2020-01-24 14:55           ` Alban Gruin
2020-01-24 18:12             ` Junio C Hamano
2020-01-24 15:05           ` [PATCH v3] " Alban Gruin
2020-01-24 18:30             ` Junio C Hamano
2020-02-05 14:31             ` Johannes Schindelin
2020-01-24 17:11           ` [PATCH v2] " Andrei Rybak
2020-01-09 11:13   ` Unreliable 'git rebase --onto' Eugeniu Rosca
     [not found] ` <CABPp-BHsyMOz+hi7EYoAnAWfzms7FRfwqCoarnu8H+vyDoN6SQ@mail.gmail.com>
2020-01-09 10:53   ` Eugeniu Rosca
2020-01-09 18:05     ` Elijah Newren
2020-01-10  0:06       ` Eugeniu Rosca
2020-01-10  2:35         ` Elijah Newren

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200109150332.GF32750@szeder.dev \
    --to=szeder.dev@gmail.com \
    --cc=avarab@gmail.com \
    --cc=erosca@de.adit-jv.com \
    --cc=git@vger.kernel.org \
    --cc=gitster@pobox.com \
    --cc=newren@gmail.com \
    --cc=peff@peff.net \
    --cc=roscaeugeniu@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Git Mailing List Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/git/0 git/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 git git/ https://lore.kernel.org/git \
		git@vger.kernel.org
	public-inbox-index git

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.git


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git