'git bisect start ...' and subsequent 'git bisect (good|bad)' 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)
[e197c21807] 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)
[565301e416] 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):
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' 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 run git
merge-base --is-ancestor HEAD $first_bad_commit' 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' 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.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
Git is a fast, scalable, distributed revision control system with an
unusually rich command set that provides both high-level operations
and full access to internals.
Git is an Open Source project covered by the GNU General Public
License version 2 (some parts of it are under different licenses,
compatible with the GPLv2). It was originally written by Linus
Torvalds with help of a group of hackers around the net.
Please read the file INSTALL for installation instructions.
Many Git online resources are accessible from https://git-scm.com/
including full documentation and Git related tools.
See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-<commandname>.txt for documentation of each command.
If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
documentation of each command with man git-<commandname> or git help <commandname>.
CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
Issues which are security relevant should be disclosed privately to
the Git Security mailing list git-security@googlegroups.com.
The maintainer frequently sends the "What's cooking" reports that
list the current status of various development topics to the mailing
list. The discussion following them give a good reference for
project status, development direction and remaining tasks.
The name "git" was given by Linus Torvalds when he wrote the very
first version. He described the tool as "the stupid content tracker"
and the name as (depending on your mood):
random three-letter combination that is pronounceable, and not
actually used by any common UNIX command. The fact that it is a
mispronunciation of "get" may or may not be relevant.
stupid. contemptible and despicable. simple. Take your pick from the
dictionary of slang.
"global information tracker": you're in a good mood, and it actually
works for you. Angels sing, and a light suddenly fills the room.
"goddamn idiotic truckload of sh*t": when it breaks
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