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Re: Revision walking, commit dates, slop




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@xxxxxxxxxxxx/
> > 
> > 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