a8dd7e05b1
Back ine37d0b8730(builtin/index-pack.c: write reverse indexes, 2021-01-25), Git learned how to read and write a pack's reverse index from a file instead of in-memory. A pack's reverse index is a mapping from pack position (that is, the order that objects appear together in a ".pack") to their position in lexical order (that is, the order that objects are listed in an ".idx" file). Reverse indexes are consulted often during pack-objects, as well as during auxiliary operations that require mapping between pack offsets, pack order, and index index. They are useful in GitHub's infrastructure, where we have seen a dramatic increase in performance when writing ".rev" files[1]. In particular: - an ~80% reduction in the time it takes to serve fetches on a popular repository, Homebrew/homebrew-core. - a ~60% reduction in the peak memory usage to serve fetches on that same repository. - a collective savings of ~35% in CPU time across all pack-objects invocations serving fetches across all repositories in a single datacenter. Reverse indexes are also beneficial to end-users as well as forges. For example, the time it takes to generate a pack containing the objects for the 10 most recent commits in linux.git (representing a typical push) is significantly faster when on-disk reverse indexes are available: $ { git rev-parse HEAD && printf '^' && git rev-parse HEAD~10 } >in $ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} pack-objects --delta-base-offset --revs --stdout <in >/dev/null' Benchmark 1: git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null Time (mean ± σ): 543.0 ms ± 20.3 ms [User: 616.2 ms, System: 58.8 ms] Range (min … max): 521.0 ms … 577.9 ms 10 runs Benchmark 2: git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null Time (mean ± σ): 245.0 ms ± 11.4 ms [User: 335.6 ms, System: 31.3 ms] Range (min … max): 226.0 ms … 259.6 ms 13 runs Summary 'git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null' ran 2.22 ± 0.13 times faster than 'git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null' The same is true of writing a pack containing the objects for the 30 most-recent commits: $ { git rev-parse HEAD && printf '^' && git rev-parse HEAD~30 } >in $ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} pack-objects --delta-base-offset --revs --stdout <in >/dev/null' Benchmark 1: git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null Time (mean ± σ): 866.5 ms ± 16.2 ms [User: 1414.5 ms, System: 97.0 ms] Range (min … max): 839.3 ms … 886.9 ms 10 runs Benchmark 2: git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null Time (mean ± σ): 581.6 ms ± 10.2 ms [User: 1181.7 ms, System: 62.6 ms] Range (min … max): 567.5 ms … 599.3 ms 10 runs Summary 'git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null' ran 1.49 ± 0.04 times faster than 'git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null' ...and savings on trivial operations like computing the on-disk size of a single (packed) object are even more dramatic: $ git rev-parse HEAD >in $ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --batch-check="%(objectsize:disk)" <in' Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in Time (mean ± σ): 305.8 ms ± 11.4 ms [User: 264.2 ms, System: 41.4 ms] Range (min … max): 290.3 ms … 331.1 ms 10 runs Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in Time (mean ± σ): 4.0 ms ± 0.3 ms [User: 1.7 ms, System: 2.3 ms] Range (min … max): 1.6 ms … 4.6 ms 1155 runs Summary 'git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in' ran 76.96 ± 6.25 times faster than 'git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in' In the more than two years sincee37d0b8730was merged, Git's implementation of on-disk reverse indexes has been thoroughly tested, both from users enabling `pack.writeReverseIndexes`, and from GitHub's deployment of the feature. The latter has been running without incident for more than two years. This patch changes Git's behavior to write on-disk reverse indexes by default when indexing a pack, which should make the above operations faster for everybody's Git installation after a repack. (The previous commit explains some potential drawbacks of using on-disk reverse indexes in certain limited circumstances, that essentially boil down to a trade-off between time to generate, and time to access. For those limited cases, the `pack.readReverseIndex` escape hatch can be used). [1]: https://github.blog/2021-04-29-scaling-monorepo-maintenance/#reverse-indexes Signed-off-by: Taylor Blau <me@ttaylorr.com> Acked-by: Derrick Stolee <derrickstolee@github.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).
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