Patrick Steinhardt
a5a82a0a20
With the move towards pluggable object databases we reorganized how we
access alternates. Before the introduction of pluggability, we were
reading objects by first scanning through packfiles from all alternates
before scanning through all loose object directories. With the change
though, we are now iterating through the sources one by one, checking
each of them for whether they contain the object either as a packed or
loose object.
Checking whether an object exists in packfiles is comparatively cheap
because the list of packs is cached, and the pack indices are likely to
be in the page cache. In contrast to that, checking whether an object
exists in loose format always incurs one stat(3p) syscall. So scanning
for objects one alternate at a time led to a performance regression in
in the case where most of the objects are contained in a packfile of an
alternate, as reported in [1].
We could fix that issue by introducing another flag that tells the
backends to only search the optimized storage backend. But that is
adding more complexity to the object database layers that is ultimately
only really required because of the way the "files" backend stores its
data. So for an alternate backend, such new infrastructure would not
really be a good fit.
But since then we have moved handling of alternates into the "files"
backend itself. One of the consequences of this is that this backend is
now having full control over all of its sub-sources, and that also makes
it way easier for us to optimize for this access pattern again.
Do so by first trying to read objects from packfiles located in any of
its object directories before trying to read loose objects.
The following benchmark demonstrates that this significantly improves
performance in some cases. The benchmark uses the Git repository itself
and sets up its object directory as an alternate, while the primary
object directory is simply an empty object directory. It then reads all
commits enumerated via `git rev-list HEAD` with git-cat-file(1):
Benchmark 1: git cat-file --batch-check --unordered (rev = HEAD~)
Time (mean ± σ): 179.4 ms ± 2.6 ms [User: 114.6 ms, System: 61.4 ms]
Range (min … max): 177.1 ms … 187.7 ms 16 runs
Benchmark 2: git cat-file --batch-check --unordered (rev = HEAD)
Time (mean ± σ): 95.0 ms ± 1.5 ms [User: 79.8 ms, System: 15.0 ms]
Range (min … max): 93.0 ms … 97.9 ms 30 runs
Summary
git cat-file --batch-check --unordered (rev = HEAD) ran
1.89 ± 0.04 times faster than git cat-file --batch-check --unordered (rev = HEAD~)
[1]: <CAFXJcxvpKHoVDwE5mBOd=w-A5vPdUmehqr8SHLUD7qv1qB00rA@mail.gmail.com>
Reported-by: Wolfgang Kritzinger <wkritzinger@atlassian.com>
Helped-by: Tyrie Vella <tyrielv@gmail.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.
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See Documentation/gittutorial.adoc to get started, then see
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If git has been correctly installed, then the tutorial can also be
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(man gitcvs-migration or git help cvs-migration if git is
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- 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