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In our production systems we have recently observed write contention in git-upload-pack(1). The system in question was consistently streaming packfiles at a rate of dozens of gigabits per second, but curiously the system was neither bottlenecked on CPU, memory or IOPS. We eventually discovered that Git was spending 80% of its time in `pipe_write()`, out of which almost all of the time was spent in the `ep_poll_callback` function in the kernel. Quoting the reporter: This infrastructure is part of an event notification queue designed to allow for multiple producers to emit events, but that concurrency safety is guarded by 3 layers of locking. The layer we're hitting contention in uses a simple reader/writer lock mode (a.k.a. shared versus exclusive mode), where producers need shared-mode (read mode), and various other actions use exclusive (write) mode. The system in question generates workloads where we have hundreds of git-upload-pack(1) processes active at the same point in time. These processes end up contending around those locks, and the consequence is that the Git processes stall. Now git-upload-pack(1) already has the infrastructure in place to buffer some of the data it reads from git-pack-objects(1) before actually sending it out. We only use this infrastructure in very limited ways though, so we generally end up matching one read(3p) call with one write(3p) call. Even worse, when the sideband is enabled we end up matching one read with _two_ writes: one for the pkt-line length, and one for the packfile data. Extend our use of the buffering infrastructure so that we soak up bytes until the buffer is filled up at least 2/3rds of its capacity. The change is relatively simple to implement as we already know to flush the buffer in `create_pack_file()` after git-pack-objects(1) has finished. This significantly reduces the number of write(3p) syscalls we need to do. Before this change, cloning the Linux repository resulted in around 400,000 write(3p) syscalls. With the buffering in place we only do around 130,000 syscalls. Now we could of course go even further and make sure that we always fill up the whole buffer. But this might cause an increase in read(3p) syscalls, and some tests show that this only reduces the number of write(3p) syscalls from 130,000 to 100,000. So overall this doesn't seem worth it. Helped-by: Matt Smiley <msmiley@gitlab.com> Signed-off-by: Patrick Steinhardt <ps@pks.im> 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.adoc to get started, then see
Documentation/giteveryday.adoc for a useful minimum set of commands, and
Documentation/git-<commandname>.adoc 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.adoc
(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 to git+subscribe@vger.kernel.org (see https://subspace.kernel.org/subscribing.html for details). The mailing list archives are available at https://lore.kernel.org/git/, https://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