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One of the checks done by `refs_verify_refnames_available()` is whether
any of the prefixes of a reference already exists. For example, given a
reference "refs/heads/main", we'd check whether "refs/heads" or "refs"
already exist, and if so we'd abort the transaction.
When updating multiple references at once, this check is performed for
each of the references individually. Consequently, because references
tend to have common prefixes like "refs/heads/" or refs/tags/", we
evaluate the availability of these prefixes repeatedly. Naturally this
is a waste of compute, as the availability of those prefixes should in
general not change in the middle of a transaction. And if it would,
backends would notice at a later point in time.
Optimize this pattern by storing prefixes in a `strset` so that we can
trivially track those prefixes that we have already checked. This leads
to a significant speedup with the "reftable" backend when creating many
references that all share a common prefix:
Benchmark 1: update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD~)
Time (mean ± σ): 63.1 ms ± 1.8 ms [User: 41.0 ms, System: 21.6 ms]
Range (min … max): 60.6 ms … 69.5 ms 38 runs
Benchmark 2: update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD)
Time (mean ± σ): 40.0 ms ± 1.3 ms [User: 29.3 ms, System: 10.3 ms]
Range (min … max): 38.1 ms … 47.3 ms 61 runs
Summary
update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD) ran
1.58 ± 0.07 times faster than update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD~)
For the "files" backend we see an improvement, but a much smaller one:
Benchmark 1: update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD~)
Time (mean ± σ): 395.8 ms ± 5.3 ms [User: 63.6 ms, System: 330.5 ms]
Range (min … max): 387.0 ms … 404.6 ms 10 runs
Benchmark 2: update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD)
Time (mean ± σ): 386.0 ms ± 4.0 ms [User: 51.5 ms, System: 332.8 ms]
Range (min … max): 380.8 ms … 392.6 ms 10 runs
Summary
update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD) ran
1.03 ± 0.02 times faster than update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD~)
This change also leads to a modest improvement when writing references
with "initial" semantics, for example when migrating references. The
following benchmarks are migrating 1m references from the "reftable" to
the "files" backend:
Benchmark 1: migrate reftable:files (refcount = 1000000, revision = HEAD~)
Time (mean ± σ): 836.6 ms ± 5.6 ms [User: 645.2 ms, System: 185.2 ms]
Range (min … max): 829.6 ms … 845.9 ms 10 runs
Benchmark 2: migrate reftable:files (refcount = 1000000, revision = HEAD)
Time (mean ± σ): 759.8 ms ± 5.1 ms [User: 574.9 ms, System: 178.9 ms]
Range (min … max): 753.1 ms … 768.8 ms 10 runs
Summary
migrate reftable:files (refcount = 1000000, revision = HEAD) ran
1.10 ± 0.01 times faster than migrate reftable:files (refcount = 1000000, revision = HEAD~)
And vice versa:
Benchmark 1: migrate files:reftable (refcount = 1000000, revision = HEAD~)
Time (mean ± σ): 870.7 ms ± 5.7 ms [User: 735.2 ms, System: 127.4 ms]
Range (min … max): 861.6 ms … 883.2 ms 10 runs
Benchmark 2: migrate files:reftable (refcount = 1000000, revision = HEAD)
Time (mean ± σ): 799.1 ms ± 8.5 ms [User: 661.1 ms, System: 130.2 ms]
Range (min … max): 787.5 ms … 812.6 ms 10 runs
Summary
migrate files:reftable (refcount = 1000000, revision = HEAD) ran
1.09 ± 0.01 times faster than migrate files:reftable (refcount = 1000000, revision = HEAD~)
The impact here is significantly smaller given that we don't perform any
reference reads with "initial" semantics, so the speedup only comes from
us doing less string list lookups.
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.
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See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
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If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
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CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
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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