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@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(). > > -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).
