b1df3b3867
In7a5d604443(commit: detect commits that exist in commit-graph but not in the ODB, 2023-10-31), we have introduced a new object existence check into `repo_parse_commit_internal()` so that we do not parse commits via the commit-graph that don't have a corresponding object in the object database. This new check of course comes with a performance penalty, which the commit put at around 30% for `git rev-list --topo-order`. But there are in fact scenarios where the performance regression is even higher. The following benchmark against linux.git with a fully-build commit-graph: Benchmark 1: git.v2.42.1 rev-list --count HEAD Time (mean ± σ): 658.0 ms ± 5.2 ms [User: 613.5 ms, System: 44.4 ms] Range (min … max): 650.2 ms … 666.0 ms 10 runs Benchmark 2: git.v2.43.0-rc1 rev-list --count HEAD Time (mean ± σ): 1.333 s ± 0.019 s [User: 1.263 s, System: 0.069 s] Range (min … max): 1.302 s … 1.361 s 10 runs Summary git.v2.42.1 rev-list --count HEAD ran 2.03 ± 0.03 times faster than git.v2.43.0-rc1 rev-list --count HEAD While it's a noble goal to ensure that results are the same regardless of whether or not we have a potentially stale commit-graph, taking twice as much time is a tough sell. Furthermore, we can generally assume that the commit-graph will be updated by git-gc(1) or git-maintenance(1) as required so that the case where the commit-graph is stale should not at all be common. With that in mind, default-disable GIT_COMMIT_GRAPH_PARANOIA and restore the behaviour and thus performance previous to the mentioned commit. In order to not be inconsistent, also disable this behaviour by default in `lookup_commit_in_graph()`, where the object existence check has been introduced right at its inception viaf559d6d45e(revision: avoid hitting packfiles when commits are in commit-graph, 2021-08-09). This results in another speedup in commands that end up calling this function, even though it's less pronounced compared to the above benchmark. The following has been executed in linux.git with ~1.2 million references: Benchmark 1: GIT_COMMIT_GRAPH_PARANOIA=true git rev-list --all --no-walk=unsorted Time (mean ± σ): 2.947 s ± 0.003 s [User: 2.412 s, System: 0.534 s] Range (min … max): 2.943 s … 2.949 s 3 runs Benchmark 2: GIT_COMMIT_GRAPH_PARANOIA=false git rev-list --all --no-walk=unsorted Time (mean ± σ): 2.724 s ± 0.030 s [User: 2.207 s, System: 0.514 s] Range (min … max): 2.704 s … 2.759 s 3 runs Summary GIT_COMMIT_GRAPH_PARANOIA=false git rev-list --all --no-walk=unsorted ran 1.08 ± 0.01 times faster than GIT_COMMIT_GRAPH_PARANOIA=true git rev-list --all --no-walk=unsorted So whereas7a5d604443initially introduced the logic to start doing an object existence check in `repo_parse_commit_internal()` by default, the updated logic will now instead cause `lookup_commit_in_graph()` to stop doing the check by default. This behaviour continues to be tweakable by the user via the GIT_COMMIT_GRAPH_PARANOIA environment variable. Note that this requires us to amend some tests to manually turn on the paranoid checks again. This is because we cause repository corruption by manually deleting objects which are part of the commit graph already. These circumstances shouldn't usually happen in repositories. Reported-by: Jeff King <peff@peff.net> Signed-off-by: Patrick Steinhardt <ps@pks.im> 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).
The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission and Documentation/CodingGuidelines).
Those wishing to help with error message, usage and informational message
string translations (localization l10) should see po/README.md
(a po file is a Portable Object file that holds the translations).
To subscribe to the list, send an email with just "subscribe git" in the body to majordomo@vger.kernel.org (not the Git list). The mailing list archives are available at https://lore.kernel.org/git/, http://marc.info/?l=git and other archival sites.
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