From: Mike Hommey <mh@glandium.org>
To: "SZEDER Gábor" <szeder.dev@gmail.com>
Cc: git@vger.kernel.org
Subject: Re: Revision walking, commit dates, slop
Date: Sat, 18 May 2019 12:58:28 +0900 [thread overview]
Message-ID: <20190518035828.pjaqfrkkvldhri6v@glandium.org> (raw)
In-Reply-To: <20190518015005.GA951@szeder.dev>
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().
Mike
next prev parent reply other threads:[~2019-05-18 3:58 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-18 0:54 Revision walking, commit dates, slop Mike Hommey
2019-05-18 1:50 ` SZEDER Gábor
2019-05-18 3:58 ` Mike Hommey [this message]
2019-05-18 4:17 ` Mike Hommey
2019-05-18 12:01 ` SZEDER Gábor
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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