c0c9d35e27
When `show-ref` is combined with the `--heads` or `--tags` options, it
can avoid iterating parts of a repository's references that it doesn't
care about.
But it doesn't take advantage of this potential optimization. When this
command was introduced back in 358ddb62cf (Add "git show-ref" builtin
command, 2006-09-15), `for_each_ref_in()` did exist. But since most
repositories don't have many (any?) references that aren't branches or
tags already, this makes little difference in practice.
Though for repositories with a large imbalance of branches and tags (or,
more likely in the case of server operators, many hidden references),
this can make quite a difference. Take, for example, a repository with
500,000 "hidden" references (all of the form "refs/__hidden__/N"), and
a single branch:
git commit --allow-empty -m "base" &&
seq 1 500000 | sed 's,\(.*\),create refs/__hidden__/\1 HEAD,' |
git update-ref --stdin &&
git pack-refs --all
Outputting the existence of that single branch currently takes on the
order of ~50ms on my machine. The vast majority of this time is wasted
iterating through references that we know we're going to discard.
Instead, teach `show-ref` that it can iterate just "refs/heads" and/or
"refs/tags" when given `--heads` and/or `--tags`, respectively. A few
small interesting things to note:
- When given either option, we can avoid the general-purpose
for_each_ref() call altogether, since we know that it won't give us
any references that we wouldn't filter out already.
- We can make two separate calls to `for_each_fullref_in()` (and
avoid, say, the more specialized `for_each_fullref_in_prefixes()`,
since we know that the set of references enumerated by each is
disjoint, so we'll never see the same reference appear in both
calls.
- We have to use the "fullref" variant (instead of just
`for_each_branch_ref()` and `for_each_tag_ref()`), since we expect
fully-qualified reference names to appear in `show-ref`'s output.
When either of `heads_only` or `tags_only` is set, we can eliminate the
strcmp() calls in `builtin/show-ref.c::show_ref()` altogether, since we
know that `show_ref()` will never see a non-branch or tag reference.
Unfortunately, we can't use `for_each_fullref_in_prefixes()` to enhance
`show-ref`'s pattern matching, since `show-ref` patterns match on the
_suffix_ (e.g., the pattern "foo" shows "refs/heads/foo",
"refs/tags/foo", and etc, not "foo/*").
Nonetheless, in our synthetic example above, this provides a significant
speed-up ("git" is roughly v2.36, "git.compile" is this patch):
$ hyperfine -N 'git show-ref --heads' 'git.compile show-ref --heads'
Benchmark 1: git show-ref --heads
Time (mean ± σ): 49.9 ms ± 6.2 ms [User: 45.6 ms, System: 4.1 ms]
Range (min … max): 46.1 ms … 73.6 ms 43 runs
Benchmark 2: git.compile show-ref --heads
Time (mean ± σ): 2.8 ms ± 0.4 ms [User: 1.4 ms, System: 1.2 ms]
Range (min … max): 1.3 ms … 5.6 ms 957 runs
Summary
'git.compile show-ref --heads' ran
18.03 ± 3.38 times faster than 'git show-ref --heads'
Signed-off-by: Taylor Blau <me@ttaylorr.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