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In cases where we expect to ask has_sha1_file() about a lot of objects that we are not likely to have (e.g., during fetch negotiation), we already use OBJECT_INFO_QUICK to sacrifice accuracy (due to racing with a simultaneous write or repack) for speed (we avoid re-scanning the pack directory). However, even checking for loose objects can be expensive, as we will stat() each one. On many systems this cost isn't too noticeable, but stat() can be particularly slow on some operating systems, or due to network filesystems. Since the QUICK flag already tells us that we're OK with a slightly stale answer, we can use that as a cue to look in our in-memory cache of each object directory. That basically trades an in-memory binary search for a stat() call. Note that it is possible for this to actually be _slower_. We'll do a full readdir() to fill the cache, so if you have a very large number of loose objects and a very small number of lookups, that readdir() may end up more expensive. This shouldn't be a big deal in practice. If you have a large number of reachable loose objects, you'll already run into performance problems (which you should remedy by repacking). You may have unreachable objects which wouldn't otherwise impact performance. Usually these would go away with the prune step of "git gc", but they may be held for up to 2 weeks in the default configuration. So it comes down to how many such objects you might reasonably expect to have, how much slower is readdir() on N entries versus M stat() calls (and here we really care about the syscall backing readdir(), like getdents() on Linux, but I'll just call this readdir() below). If N is much smaller than M (a typical packed repo), we know this is a big win (few readdirs() followed by many uses of the resulting cache). When N and M are similar in size, it's also a win. We care about the latency of making a syscall, and readdir() should be giving us many values in a single call. How many? On Linux, running "strace -e getdents ls" shows a 32k buffer getting 512 entries per call (which is 64 bytes per entry; the name itself is 38 bytes, plus there are some other fields). So we can imagine that this is always a win as long as the number of loose objects in the repository is a factor of 500 less than the number of lookups you make. It's hard to auto-tune this because we don't generally know up front how many lookups we're going to do. But it's unlikely for this to perform significantly worse. 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-.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://public-inbox.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