How can I find out what commit(s) git bisect would try next?

Question

In some cases during a git bisect session, testing a particular commit takes quite long (for instance, because I have to build a full release package and deploy it on a particularly strange machine). In fact, testing a build takes so long that I'd like to start building the next two commits already without knowing whether the current test succeeds or not. That way, I can speed up bisecting by testing the current version and build the next two versions in parallel.

Does anybody know a trick to make git bisect show the next two revisions, depending on whether the current commit is good or bad?

Marley

Answer 1

git bisect uses git rev-list --bisect internally to find out, which revision is the midpoint between two revisions. You can use it by yourself to basically reimplement git bisect. It shouldn't be that hard.

Miru

Answer 2

git bisect uses git rev-list --bisect internally to find out, which revision is the midpoint between two revisions.

That has changed with Git 2.30 (Q1 2021): "git bisect start/next"^(man) in a large span of history spends a lot of time trying to come up with exactly the half-way point; this can be optimized by stopping when we see a commit that is close enough to the half-way point.

See commit 0afcea7 (12 Nov 2020) by SZEDER Gábor (szeder).
^{(Merged by Junio C Hamano -- gitster -- in commit 2557c11, 25 Nov 2020)}

bisect: loosen halfway() check for a large number of commits

^{Signed-off-by: SZEDER Gábor}

'git bisect start ...'^(man) and subsequent 'git bisect (good|bad)'^(man) commands can take quite a while when the given/remaining revision range between good and bad commits is big and contains a lot of merge commits, e.g. in git.git:

$ git rev-list --count v1.6.0..v2.28.0
44284
$ time git bisect start v2.28.0 v1.6.0
Bisecting: 22141 revisions left to test after this (roughly 15 steps)
[e197c21807dacadc8305250baa0b9228819189d4] unable_to_lock_die(): rename function from unable_to_lock_index_die()  

real    0m15.472s
user    0m15.220s
sys     0m0.255s  

The majority of the runtime is spent in do_find_bisection(), where we try to find a commit as close as possible to the halfway point between the bad and good revisions, i.e. a commit from which the number of reachable commits that are in the good-bad range is half the total number of commits in that range.
So we count how many commits are reachable in the good-bad range for each commit in that range, which is quick and easy for a linear history, even over 300k commits in a linear range are handled in ~0.3s on my machine.
Alas, handling merge commits is non-trivial and quite expensive as the algorithm used seems to be quadratic, causing the long runtime shown above.

Interestingly, look at what a big difference one additional commit can make:

$ git rev-list --count v1.6.0^..v2.28.0
44285
$ time git bisect start v2.28.0 v1.6.0^
Bisecting: 22142 revisions left to test after this (roughly 15 steps)
[565301e41670825ceedf75220f2918ae76831240] Sync with 2.1.2  

real  0m5.848s
user  0m5.600s
sys   0m0.252s  

The difference is caused by one of the optimizations attempting to cut down the runtime added in 1c4fea3a40 ("git-rev-list --bisect: optimization", 2007-03-21, Git v1.5.2-rc0 -- merge):

Another small optimization is whenever we find a half-way commit
(that is, a commit that can reach exactly half of the commits),
we stop giving counts to remaining commits, as we will not find
any better commit than we just found.  

In this second 'git bisect start'^(man) command we happen to find a commit exactly at the halfway point and can return early, but in the first case there is no such commit, so we can't return early and end up counting the number of reachable commits from all commits in the good-bad range.

However, when we have thousands of commits it's not all that important to find the exact halfway point, a few commits more or less doesn't make any real difference for the bisection.

So let's loosen the check in the halfway() helper to consider commits within about 0.1% of the exact halfway point as halfway as well, and rename the function to approx_halfway() accordingly.
This will allow us to return early on a bigger good-bad range, even when there is no commit exactly at the halfway point, thereby reducing the runtime of the first command above considerably, from ~15s to 4.901s.

Furthermore, even if there is a commit exactly at the halfway point, we might still stumble upon a commit within that 0.1% range before finding the exact halfway point, allowing us to return a bit earlier, slightly reducing the runtime of the second command from 5.848s to 5.058s.

Note that this change doesn't affect good-bad ranges containing ~2000 commits or less, because that 0.1% tolerance becomes zero due to integer arithmetic; however, if the range is that small then counting the reachable commits for all commits is already fast enough anyway.

Naturally, this will likely change which commits get picked at each bisection step, and, in turn, might change how many bisection steps are necessary to find the first bad commit.
If the number of necessary bisection steps were to increase often, then this change could backfire, because building and testing at each step might take much longer than the time spared.
OTOH, if the number of steps were to decrease, then it would be a double win.

So I ran some tests to see how often that happens: picked random good and bad starting revisions at least 50k commits apart and a random first bad commit in between in git.git, and used 'git bisect git merge-base run --is-ancestor HEAD $first_bad_commit'^(man) to check the number of necessary bisection steps.
After repeating all this 1000 times both with and without this patch I found that:

• 146 cases needed one more bisection step than before, 149 cases needed one less step, while in the remaining 705 cases the number of steps didn't change.
  So the number of bisection steps does indeed change in a non-negligible number of cases, but it seems that the average number of steps doesn't change in the long run.
• The first 'git bisect start'^(man) command got over 3x faster in 456 cases, so this "no commit at the exact halfway point" case seems to be common enough to care about.

Unrequited