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From: "SZEDER Gábor" <szeder.dev@gmail.com>
To: Mike Hommey <mh@glandium.org>
Cc: git@vger.kernel.org, Kacper Kornet <draenog@pld-linux.org>
Subject: Re: Revision walking, commit dates, slop
Date: Sat, 18 May 2019 14:01:40 +0200	[thread overview]
Message-ID: <20190518120140.GB951@szeder.dev> (raw)
In-Reply-To: <20190518041706.ct6ie5trvxgdhjar@glandium.org>

On Sat, May 18, 2019 at 01:17:06PM +0900, Mike Hommey wrote:
> On Sat, May 18, 2019 at 12:58:28PM +0900, Mike Hommey wrote:
> > On Sat, May 18, 2019 at 03:50:05AM +0200, SZEDER Gábor wrote:
> > > On Sat, May 18, 2019 at 09:54:12AM +0900, Mike Hommey wrote:
> > > > There are established corner cases, where in a repo where commit dates
> > > > are not monotonically increasing, revision walking can go horribly
> > > > wrong. This was discussed in the past in e.g.
> > > > https://public-inbox.org/git/20150521061553.GA29269@glandium.org/
> > > > 
> > > > The only (simple) workable way, given the current algorithm, to get an
> > > > accurate view off rev-list is to essentially make slop infinite. This
> > > > works fine, at the expense of runtime.
> > > > 
> > > > Now, ignoring any modification for the above, I'm hitting another corner
> > > > case in some other "weird" history, where I have 500k commits all with
> > > > the same date. With such a commit dag, something as trivial as
> > > > `git rev-list HEAD~..HEAD` goes through all commits from the root commit
> > > > to HEAD, which takes multiple seconds, when the (obvious) output is one
> > > > commit.
> > > > 
> > > > It looks like the only way revision walking stops going through all the
> > > > ancestry is through slop, and slop is essentially made infinite by the
> > > > fact all commits have the same date (because of the date check in
> > > > still_interesting(). By extension, this means the workaound for the
> > > > first corner case above, which is to make slop infinite, essentially
> > > > makes all rev walking go through the entire ancestry of the commits
> > > > given on the command line.
> > > > 
> > > > It feels like some cases of everybody_uninteresting should shorcut slop
> > > > entirely, but considering the only way for slop to decrease at all is
> > > > when everybody_uninteresting returns true, that would seem like a wrong
> > > > assumption. But I'm also not sure what slop helps with in the first
> > > > place (but I don't have a clear view of the broader picture of how the
> > > > entire revision walking works).
> > > > 
> > > > Anyways, a rather easy way to witness this happening is to create a
> > > > dummy repo like:
> > > >   git init foo
> > > >   cd foo
> > > >   for i in $(seq 1 50); do
> > > >     echo $i > a;
> > > >     git add a;
> > > >     git commit -a -m $i;
> > > >   done
> > > > 
> > > > The something as simple as `git rev-list HEAD~..HEAD` will go through
> > > > all 50 commits (assuming the script above created commits in the same
> > > > second, which it did on my machine)
> > > > 
> > > > By the time both HEAD~ and HEAD have been processed, the revision
> > > > walking should have enough information to determine that it doesn't need
> > > > to go further, but still does. Even with something like HEAD~2..HEAD,
> > > > after the first round of processing parents it should be able to see
> > > > there's not going to be any more interesting commits.
> > > > 
> > > > I'm willing to dig into this, but if someone familiar with the
> > > > algorithm could give me some hints as to what I might be missing in the
> > > > big picture, that would be helpful.
> > > 
> > > All the above is without commit-graph, I presume?  If so, then you
> > > should give it a try, as it might bring immediate help in your
> > > pathological repo.  With 5k commit in the same second (enforced via
> > > 'export GIT_COMMITTER_DATE=$(date); for i in {1..5000} ...') I get:
> > > 
> > >   $ best-of-five -q git rev-list HEAD~..HEAD
> > >   0.069
> > >   $ git commit-graph write --reachableComputing commit graph generation
> > >   numbers: 100% (5000/5000), done.
> > >   $ best-of-five -q git rev-list HEAD~..HEAD
> > >   0.004
> > 
> > I'm not observing any difference from using commit-graph, whether in
> > time or in the number of commits that are looked at in limit_list().
> 
> -c core.commitGraph=true does make a difference in time, but not in the
> number of commits looked at in limit_list(). So it's only faster because
> each iteration of the loop is faster. It means it's still dependent on
> the depth of the dag, and the larger the repo will grow, the slower it
> will get.

Oh, indeed.  Well, at least you'll waste about an order of magnitude
less processor time until you figure out how to fix it :)

Btw, once upon a time this was fast, but it became slow with commit
c19d1b4e84 (Fix revision walk for commits with the same dates,
2013-03-22).



  reply	other threads:[~2019-05-18 12:01 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-18  0:54 Mike Hommey
2019-05-18  1:50 ` SZEDER Gábor
2019-05-18  3:58   ` Mike Hommey
2019-05-18  4:17     ` Mike Hommey
2019-05-18 12:01       ` SZEDER Gábor [this message]
2019-05-19 22:28         ` Jakub Narebski
2019-05-20  1:33       ` Derrick Stolee
2019-05-20 11:02         ` Jakub Narebski
2019-05-20 11:20           ` Derrick Stolee
2019-05-20 13:42             ` Jakub Narebski
2019-05-20 23:27               ` Jakub Narebski
2019-05-21  1:20                 ` Derrick Stolee
2019-05-22 18:29                   ` Jakub Narebski
2019-05-22 19:06                     ` Derrick Stolee
2019-05-23 21:04                       ` Jakub Narebski
2019-06-25  7:51               ` Jakub Narebski
2019-06-25 10:54                 ` Derrick Stolee
2019-09-18  8:43                   ` [RFC/PATCH] commit-graph: generation v5 (backward compatible date ceiling) Jakub Narebski
2019-09-18 12:26                     ` Derrick Stolee
2019-05-21 13:14         ` [PATCH] revision: use generation for A..B --topo-order queries Derrick Stolee
2019-05-21 13:59           ` [PATCH 2/2] revision: keep topo-walk free of unintersting commits Derrick Stolee
2019-05-22  2:19             ` Mike Hommey
2019-05-21  2:00   ` Revision walking, commit dates, slop Jonathan Nieder

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