870eb53ab2
When handling --points-at, we have to try to peel each ref to see if
it's a tag that points at a requested oid. We start this process by
calling parse_object() on the oid pointed to by each ref.
The cost of parsing each object adds up, especially in an output that
doesn't otherwise need to open the objects at all. Ideally we'd use
peel_iterated_oid() here, which uses the cached information in the
packed-refs file. But we can't, because our --points-at must match not
only the fully peeled value, but any interim values (so if tag A points
to tag B which points to commit C, we should match --points-at=B, but
peel_iterated_oid() will only tell us about C).
So the best we can do (absent changes to the packed-refs peel traits) is
to avoid parsing non-tags. The obvious way to do that is to call
oid_object_info() to check the type before parsing. But there are a few
gotchas there, like checking if the object has already been parsed.
Instead we can just tell parse_object() that we are OK skipping the hash
check, which lets it turn on several optimizations. Commits can be
loaded via the commit graph (so it's both fast and we have the benefit
of the parsed data if we need it later at the output stage). Blobs are
not loaded at all. Trees are still loaded, but it's rather rare to have
a ref point directly to a tree (and since this is just an optimization,
kicking in 99% of the time is OK).
Even though we're paying for an extra lookup, the cost to avoid parsing
the non-tags is a net benefit. In my git.git repository with 941 tags
and 1440 other refs pointing to commits, this significantly cuts the
runtime:
Benchmark 1: ./git.old for-each-ref --points-at=HEAD
Time (mean ± σ): 26.8 ms ± 0.5 ms [User: 24.5 ms, System: 2.2 ms]
Range (min … max): 25.9 ms … 29.2 ms 107 runs
Benchmark 2: ./git.new for-each-ref --points-at=HEAD
Time (mean ± σ): 9.1 ms ± 0.3 ms [User: 6.8 ms, System: 2.2 ms]
Range (min … max): 8.6 ms … 10.2 ms 308 runs
Summary
'./git.new for-each-ref --points-at=HEAD' ran
2.96 ± 0.10 times faster than './git.old for-each-ref --points-at=HEAD'
In a repository that is mostly annotated tags, we'd expect less
improvement (we might still skip a few object loads, but that's balanced
by the extra lookups). In my clone of linux.git, which has 782 tags and
3 branches, the run-time is about the same (it's actually ~1% faster on
average after this patch, but that's within the run-to-run noise).
Signed-off-by: Jeff King <peff@peff.net>
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