5 ms·

The magic of bisect is that you rule out half of your remaining commits every time you run it. So even if you have 1000 commits, it takes at most 10 runs. An n-

The magic of bisect is that you rule out half of your remaining commits every time you run it. So even if you have 1000 commits, it takes at most 10 runs. An n-bisect wouldn't be that much faster, it could be slower because you will not always be able to rule out half your commits.

actionfromafar 2y ago
Yes, but I could also see the case where you have 10 commits to check, each bisect takes 20 minutes and it takes 40 minutes to find the problem. Or 20 minutes if you had 10-'sect.
LegionMammal978 2y ago
The idea is, suppose I did a trisect, splitting the range [start,end) into [start,A), [A,B), and [B,end). At each step, I test commits A and B in parallel. If both A and B are bad, I continue with [start,A). If A is good and B is bad, I continue with [A,B). If both A and B are good, I continue with [B,end). This lets me rule out two thirds of the commits, in the same time that an ordinary bisect would have ruled out half. (I'm assuming that the tests don't benefit from having additional cores available.) In general, for an n-sect, you'd test n - 1 commits in parallel, and divide the number of remaining commits by n each time.
- agumonkey 2y ago
  have you ever seen this implemented somewhere ? interesting idea
  - LegionMammal978 2y ago
    No, unfortunately not. If your history is strictly linear, you could probably hack together something relatively simple on top of git rev-list. But git bisect does all sorts of magic to deal with merge commits and other funny situations, and generalizing that to an n-sect would take a fair bit of work.
rcxdude 2y ago
Yes, you'd need 4x parallelism for a 2x speedup (16x for 4x, etc). But there's plenty of situations where that would be practical and worthwhile (think a build and test cycle that takes ~1 hour each and can't be meaningfully parallelised further).

5 ms·

The magic of bisect is that you rule out half of your remaining commits every time you run it. So even if you have 1000 commits, it takes at most 10 runs. An n-

by andmarios 2y ago

actionfromafar 2y ago
Yes, but I could also see the case where you have 10 commits to check, each bisect takes 20 minutes and it takes 40 minutes to find the problem. Or 20 minutes if you had 10-'sect.
LegionMammal978 2y ago
The idea is, suppose I did a trisect, splitting the range [start,end) into [start,A), [A,B), and [B,end). At each step, I test commits A and B in parallel. If both A and B are bad, I continue with [start,A). If A is good and B is bad, I continue with [A,B). If both A and B are good, I continue with [B,end). This lets me rule out two thirds of the commits, in the same time that an ordinary bisect would have ruled out half. (I'm assuming that the tests don't benefit from having additional cores available.) In general, for an n-sect, you'd test n - 1 commits in parallel, and divide the number of remaining commits by n each time.
- agumonkey 2y ago
  have you ever seen this implemented somewhere ? interesting idea
  - LegionMammal978 2y ago
    No, unfortunately not. If your history is strictly linear, you could probably hack together something relatively simple on top of git rev-list. But git bisect does all sorts of magic to deal with merge commits and other funny situations, and generalizing that to an n-sect would take a fair bit of work.
rcxdude 2y ago
Yes, you'd need 4x parallelism for a 2x speedup (16x for 4x, etc). But there's plenty of situations where that would be practical and worthwhile (think a build and test cycle that takes ~1 hour each and can't be meaningfully parallelised further).