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Patrick Steinhardt 0a0fbbe3ff refs: remove lookup cache for reference-transaction hook 
When adding the reference-transaction hook, there were concerns about
the performance impact it may have on setups which do not make use of
the new hook at all. After all, it gets executed every time a reftx is
prepared, committed or aborted, which linearly scales with the number of
reference-transactions created per session. And as there are code paths
like `git push` which create a new transaction for each reference to be
updated, this may translate to calling `find_hook()` quite a lot.

To address this concern, a cache was added with the intention to not
repeatedly do negative hook lookups. Turns out this cache caused a
regression, which was fixed via e5256c82e5 (refs: fix interleaving hook
calls with reference-transaction hook, 2020-08-07). In the process of
discussing the fix, we realized that the cache doesn't really help even
in the negative-lookup case. While performance tests added to benchmark
this did show a slight improvement in the 1% range, this really doesn't
warrent having a cache. Furthermore, it's quite flaky, too. E.g. running
it twice in succession produces the following results:

Test                         master            pks-reftx-hook-remove-cache
--------------------------------------------------------------------------
1400.2: update-ref           2.79(2.16+0.74)   2.73(2.12+0.71) -2.2%
1400.3: update-ref --stdin   0.22(0.08+0.14)   0.21(0.08+0.12) -4.5%

Test                         master            pks-reftx-hook-remove-cache
--------------------------------------------------------------------------
1400.2: update-ref           2.70(2.09+0.72)   2.74(2.13+0.71) +1.5%
1400.3: update-ref --stdin   0.21(0.10+0.10)   0.21(0.08+0.13) +0.0%

One case notably absent from those benchmarks is a single executable
searching for the hook hundreds of times, which is exactly the case for
which the negative cache was added. p1400.2 will spawn a new update-ref
for each transaction and p1400.3 only has a single reference-transaction
for all reference updates. So this commit adds a third benchmark, which
performs an non-atomic push of a thousand references. This will create a
new reference transaction per reference. But even for this case, the
negative cache doesn't consistently improve performance:

Test                         master            pks-reftx-hook-remove-cache
--------------------------------------------------------------------------
1400.4: nonatomic push       6.63(6.50+0.13)   6.81(6.67+0.14) +2.7%
1400.4: nonatomic push       6.35(6.21+0.14)   6.39(6.23+0.16) +0.6%
1400.4: nonatomic push       6.43(6.31+0.13)   6.42(6.28+0.15) -0.2%

So let's just remove the cache altogether to simplify the code.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
 		2020-08-25 15:34:42 -07:00
.github Merge branch 'es/advertise-contribution-doc' 2020-06-17 21:54:06 -07:00
Documentation githooks.txt: use correct "reference-transaction" hook name 2020-07-24 13:53:58 -07:00
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ci Merge branch 'jk/ci-only-on-selected-branches' 2020-05-29 15:12:19 -07:00
compat Merge branch 'js/msvc-build-fix' 2020-06-17 21:54:03 -07:00
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t refs: remove lookup cache for reference-transaction hook 2020-08-25 15:34:42 -07:00
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README.md

Build status

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