2d53975488
name_rev() assigns a name to a commit and its parents and grandparents and so on. Commits share their name string with their first parent, which in turn does the same, recursively to the root. That saves a lot of allocations. When a better name is found, the old name is replaced, but its memory is not released. That leakage can become significant. Can we release these old strings exactly once even though they are referenced multiple times? Yes, indeed -- we can make use of the fact that name_rev() visits the ancestors of a commit after it set a new name for it and tries to update their names as well. Members of the first ancestral line have the same taggerdate and from_tag values, but a higher distance value than their child commit at generation 0. These are the only criteria used by is_better_name(). Lower distance values are considered better, so a name that is better for a child will also be better for its parent and grandparent etc. That means we can free(3) an inferior name at generation 0 and rely on name_rev() to replace all references in ancestors as well. If we do that then we need to stop using the string pointer alone to distinguish new empty rev_name slots from initialized ones, though, as it technically becomes invalid after the free(3) call -- even though its value is still different from NULL. We can check the generation value first, as empty slots will have it initialized to 0, and for the actual generation 0 we'll set a new valid name right after the create_or_update_name() call that releases the string. For the Chromium repo, releasing superceded names reduces the memory footprint of name-rev --all significantly. Here's the output of GNU time before: 0.98user 0.48system 0:01.46elapsed 99%CPU (0avgtext+0avgdata 2601812maxresident)k 0inputs+0outputs (0major+571470minor)pagefaults 0swaps ... and with this patch: 1.01user 0.26system 0:01.28elapsed 100%CPU (0avgtext+0avgdata 1559196maxresident)k 0inputs+0outputs (0major+314370minor)pagefaults 0swaps It also gets faster; hyperfine before: Benchmark #1: ./git -C ../chromium/src name-rev --all Time (mean ± σ): 1.534 s ± 0.006 s [User: 1.039 s, System: 0.494 s] Range (min … max): 1.522 s … 1.542 s 10 runs ... and with this patch: Benchmark #1: ./git -C ../chromium/src name-rev --all Time (mean ± σ): 1.338 s ± 0.006 s [User: 1.047 s, System: 0.291 s] Range (min … max): 1.327 s … 1.346 s 10 runs For the Linux repo it doesn't pay off; memory usage only gets down from: 0.76user 0.03system 0:00.80elapsed 99%CPU (0avgtext+0avgdata 292848maxresident)k 0inputs+0outputs (0major+44579minor)pagefaults 0swaps ... to: 0.78user 0.03system 0:00.81elapsed 100%CPU (0avgtext+0avgdata 284696maxresident)k 0inputs+0outputs (0major+44892minor)pagefaults 0swaps The runtime actually increases slightly from: Benchmark #1: ./git -C ../linux/ name-rev --all Time (mean ± σ): 828.8 ms ± 5.0 ms [User: 797.2 ms, System: 31.6 ms] Range (min … max): 824.1 ms … 838.9 ms 10 runs ... to: Benchmark #1: ./git -C ../linux/ name-rev --all Time (mean ± σ): 847.6 ms ± 3.4 ms [User: 807.9 ms, System: 39.6 ms] Range (min … max): 843.4 ms … 854.3 ms 10 runs Why is that? In the Chromium repo, ca. 44000 free(3) calls in create_or_update_name() release almost 1GB, while in the Linux repo 240000+ calls release a bit more than 5MB, so the average discarded name is ca. 1000x longer in the latter. Overall I think it's the right tradeoff to make, as it helps curb the memory usage in repositories with big discarded names, and the added overhead is small. Signed-off-by: René Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com> |
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| INSTALL | ||
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README.md
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). To subscribe to the list, send an email with just "subscribe git" in the body to majordomo@vger.kernel.org. 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