1f34bf3e08
The multi-pack index acts as a cache across a set of packfiles so that
we can quickly look up which of those packfiles contains a given object.
As such, the multi-pack index naturally needs to be updated every time
one of the packfiles goes away, or otherwise the multi-pack index has
grown stale.
A stale multi-pack index should be handled gracefully by Git though, and
in fact it is: if the indexed pack cannot be found we simply ignore it
and eventually we fall back to doing the object lookup by just iterating
through all packs, even if those aren't indexed.
But while this fallback works, it has one significant downside: we don't
cache the fact that a pack has vanished. This leads to us repeatedly
trying to look up the same pack only to realize that it (still) doesn't
exist.
This issue can be easily demonstrated by creating a repository with a
stale multi-pack index and a couple of objects. We do so by creating a
repository with two packfiles, both of which are indexed by the
multi-pack index, and then repack those two packfiles. Note that we have
to move the multi-pack-index before doing the final repack, as Git knows
to delete it otherwise.
$ git init repo
$ cd repo/
$ git config set maintenance.auto false
$ for i in $(seq 1000); do printf "%d-original" $i >file-$i; done
$ git add .
$ git commit -moriginal
$ git repack -dl
$ for i in $(seq 1000); do printf "%d-modified" $i >file-$i; done
$ git commit -a -mmodified
$ git repack -dl
$ git multi-pack-index write
$ mv .git/objects/pack/multi-pack-index .
$ git repack -Adl
$ mv multi-pack-index .git/objects/pack/
Commands that cause a lot of objects lookups will now repeatedly invoke
`add_packed_git()`, which leads to three failed access(3p) calls as well
as one failed stat(3p) call. The following strace for example is done
for `git log --patch` in the above repository:
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
74.67 0.024693 1 18038 18031 access
25.33 0.008378 1 6045 6017 newfstatat
------ ----------- ----------- --------- --------- ----------------
100.00 0.033071 1 24083 24048 total
Fix the issue by introducing a negative lookup cache for indexed packs.
This cache works by simply storing an invalid pointer for a missing pack
when `prepare_midx_pack()` fails to look up the pack. Most users of the
`packs` array don't need to be adjusted, either, as they all know to
call `prepare_midx_pack()` before accessing the array.
With this change in place we can now see a significantly reduced number
of syscalls:
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
73.58 0.000323 5 60 28 newfstatat
26.42 0.000116 5 23 16 access
------ ----------- ----------- --------- --------- ----------------
100.00 0.000439 5 83 44 total
Furthermore, this change also results in a speedup:
Benchmark 1: git log --patch (revision = HEAD~)
Time (mean ± σ): 50.4 ms ± 2.5 ms [User: 22.0 ms, System: 24.4 ms]
Range (min … max): 45.4 ms … 54.9 ms 53 runs
Benchmark 2: git log --patch (revision = HEAD)
Time (mean ± σ): 12.7 ms ± 0.4 ms [User: 11.1 ms, System: 1.6 ms]
Range (min … max): 12.4 ms … 15.0 ms 191 runs
Summary
git log --patch (revision = HEAD) ran
3.96 ± 0.22 times faster than git log --patch (revision = HEAD~)
In the end, it should in theory never be necessary to have this negative
lookup cache given that we know to update the multi-pack index together
with repacks. But as the change is quite contained and as the speedup
can be significant as demonstrated above, it does feel sensible to have
the negative lookup cache regardless.
Based-on-patch-by: Jeff King <peff@peff.net>
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