"David Symonds" <dsymonds@xxxxxxxxx> writes: > Git is used to track more than just source code that can be "built", > and git bisect can be used for more than just tracking down bugs. > > I'm not convinced the considerable extra complexity would be > worthwhile. You'd have to have git bisect do checkouts to new > temporary directories, track them and clean them up. I personally think "git bisect" Porcelain does a bit too much. For example, it always checks the revision out, but some non-build checks can be done without having a checkout. The core idea of bisect, which is really about how to effectively use "git rev-list --bisect", is quite simple. You start by one bad and zero or more "good" ones, where the "bad" one is a descendant of the good ones, and hunt for a _single_ change that changed a "good" state into a "bad" one. This last point is important. The only thing "bisect" can find is a single flip where all of its ancestors are "good" and where all of its descendants are "bad". Think of a bad gene introduced by a mutation at a particular commit (i.e. "generation") and that contaminates all of its children and descendants forever. If you visualize the commit ancestry graph growing from left to right like we usually draw in our documentation and e-mails, and if you paint known-to-be-good ones blue, known-to-be-bad ones red, and unknown ones yellow, you will get a picture not unlike the ones shown on pp.111-115 of http://userweb.kernel.org/~junio/200607-ols.pdf The underlying "git rev-list --bisect" takes a set of "good" commits and a single "bad" commit, computes the set of commits that are unknown ("yellow"), and gives one of them that lies halfway from "good" ones ("blue") and the "bad" one ("red"). The way "git bisect" operates is: (0) Prime the process by getting a single "bad" and zero or more "good"; switch to "bisect" branch. (1) Ask "rev-list --bisect" the midpoint; check out that revision for you to test. (2) It's your turn to give more information to "git bisect". (2-a) If you say "good", it is added to the set of "good"; go back to (1) (2-b) If you say "bad", it is set to "bad" (because of the way bisection works, this is always an ancestor of the previous "bad", and because the only thing we do is to find a single flip, keeping a single "bad" that is an ancestor of all the commits previously known-to-be-bad is sufficient); go back to (1) One thing to note is that in (2-a) or (2-b), you do not necessarily have to say the commit the command gave you in step (1) is good or bad. If the revision given by (1) is untestable, you can reset to another yellow one, test that, and tell the command "This is good/bad". So one way to speed up your bisection process would be: * Have multiple work trees (e.g. contrib/workdir/git-new-workdir); * Run bisect in one repository; * In step (1) of each round, look at gitk output and pick another commit that is distant from the one you are going to test. In another work tree, check that one out and test it in parallel. * You can feed the good/bad information you obtained from the test you run in the neighbouring work tree, in addition to what you learn in your main tree, with "git bisect good $it" or "git bisect bad $it". > It might be interesting if you approached it as a tri-section or a > general N-section where you try to divide the interval into N parts > and concurrently test N-1 commits. But really, do you find git bisect > all that slow in practice? You probably have a reasonable guess as to > where a regression has come in, and so even 1000 revisions needs at > most 10 bisections to find the culprit. I think adding N-section to "rev-list --bisect" would generally be an interesting thing to do. For one thing, it would allow you to automate the step to "pick another commit that is distant from the one you are going to test" in the above sequence. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
