488ae8cf26
The original design of changed-path Bloom filters included an 8-byte
block size for filter lengths. This was changed mid-way through the
submission process, and now the length stored in the commit-graph has
one-byte granularity.
This can cause some issues for very small filters. The analysis for
false positive rates assume large filters, so rounding errors become
less important at that scale. When there are only a few paths changed,
a filter that has size only a few bytes could have very different
behavior. In fact, this is evidenced in the Git repository due to the
code organization and careful patch creation that leads to many commits
with very small filters. These small filters frequently have
false-positive rates in the 8-10% range or higher.
The previous change improved the false-positive rate using multiple
Bloom keys when the path has multiple directory components. However,
that does not help at all for files at root. It is typical to have
several commits that change only the README at root, and those commits
would be likely to have these artificially high false-positive rates.
Correct this issue by creating a minimum filters size of 8 bytes. This
requires the very small commits (with fewer than six changes, including
non-root directories) to have a larger filter. In principle, this
violates the bits_per_entry value of struct bloom_filter_settings.
However, it does not actually create a functional problem.
As for compatibility, this only affects new versions writing filters for
commits that do not yet have a filter. Old version will write the
smaller filters and this version will persist and properly read that
data. Now, the new files will be generated slightly larger.
Bytes before Bytes after Difference
--------------------------------------------------
git 4,021,078 4,275,311 +6.32%
linux 72,212,101 73,909,286 +2.35%
tensorflow 7,596,359 7,691,646 +1.25%
This has a measurable improvement in the false-positive rate and the
end-to-end run time for these repos. The table below compares the average
false-positive rate and runtime of
git rev-list HEAD -- "$path"
before and after this change for 5000+ randomly* selected paths from
each repository:
Average false Average Average
positive rate runtime runtime
before after before after difference
------------------------------------------------------------------
git 0.786% 0.227% 0.0387s 0.0289s -25.5%
linux 0.0296% 0.0174% 0.0766s 0.0706s -7.8%
tensorflow 0.6977% 0.0268% 0.0420s 0.0384s -8.5%
*Path selection was done with the following pipeline:
git ls-tree -r --name-only HEAD | sort -R | head -n 5000
These relatively-small increases in file size appear to be a fair price
to pay for these performance improvements.
Signed-off-by: Derrick Stolee <dstolee@microsoft.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). 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