fffd981ec2
Records store their keys prefix-compressed. As many records will share a common prefix (e.g. "refs/heads/"), this can end up saving quite a bit of disk space. The downside of this is that it is not possible to just seek into the middle of a block and consume the corresponding record because it may depend on prefixes read from preceding records. To help with this usecase, the reftable format writes every n'th record without using prefix compression, which is called a "restart". The list of restarts is stored at the end of each block so that a reader can figure out entry points at which to read a full record without having to read all preceding records. This allows us to do a binary search over the records in a block when searching for a particular key by iterating through the restarts until we have found the section in which our record must be located. From thereon we perform a linear search to locate the desired record. This mechanism is broken though. In `block_reader_seek()` we call `binsearch()` over the count of restarts in the current block. The function we pass to compare records with each other computes the key at the current index and then compares it to our search key by calling `strbuf_cmp()`, returning its result directly. But `binsearch()` expects us to return a truish value that indicates whether the current index is smaller than the searched-for key. And unless our key exactly matches the value at the restart counter we always end up returning a truish value. The consequence is that `binsearch()` essentially always returns 0, indicacting to us that we must start searching right at the beginning of the block. This works by chance because we now always do a linear scan from the start of the block, and thus we would still end up finding the desired record. But needless to say, this makes the optimization quite useless. Fix this bug by returning whether the current key is smaller than the searched key. As the current behaviour was correct it is not possible to write a test. Furthermore it is also not really possible to demonstrate in a benchmark that this fix speeds up seeking records. This may cause the reader to question whether this binary search makes sense in the first place if it doesn't even help with performance. But it would end up helping if we were to read a reftable with a much larger block size. Blocks can be up to 16MB in size, in which case it will become much more important to avoid the linear scan. We are not yet ready to read or write such larger blocks though, so we have to live without a benchmark demonstrating this. 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.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 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