On Wed, Feb 19, 2025 at 02:23:43PM +0100, Patrick Steinhardt wrote: > When verifying whether refnames are available we have to verify whether > any reference exists that is nested under the current reference. E.g. > given a reference "refs/heads/foo", we must make sure that there is no > other reference "refs/heads/foo/*". > > This check is performed using a ref iterator with the prefix set to the > nested reference namespace. Until now it used to not be possible to > reseek iterators, so we always had to reallocate the iterator for every > single reference we're about to check. This keeps us from reusing state > that the iterator may have and that may make it work more efficiently. > > Refactor the logic to reseek iterators. This leads to a sizeable speedup > with the "reftable" backend: > > Benchmark 1: update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD~) > Time (mean ± σ): 39.8 ms ± 0.9 ms [User: 29.7 ms, System: 9.8 ms] > Range (min … max): 38.4 ms … 42.0 ms 62 runs > > Benchmark 2: update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD) > Time (mean ± σ): 31.9 ms ± 1.1 ms [User: 27.0 ms, System: 4.5 ms] > Range (min … max): 29.8 ms … 34.3 ms 74 runs > > Summary > update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD) ran > 1.25 ± 0.05 times faster than update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD~) > > The "files" backend doesn't really show a huge impact: > > Benchmark 1: update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD~) > Time (mean ± σ): 392.3 ms ± 7.1 ms [User: 59.7 ms, System: 328.8 ms] > Range (min … max): 384.6 ms … 404.5 ms 10 runs > > Benchmark 2: update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD) > Time (mean ± σ): 387.7 ms ± 7.4 ms [User: 54.6 ms, System: 329.6 ms] > Range (min … max): 377.0 ms … 397.7 ms 10 runs > > Summary > update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD) ran > 1.01 ± 0.03 times faster than update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD~) > > This is mostly because it is way slower to begin with because it has to > create a separate file for each new reference, so the milliseconds we > shave off by reseeking the iterator doesn't really translate into a > significant relative improvement. Interesting, because there are many I/O operations which hides the compute latency. Even though we improve the compute speed, the I/O operations would still delay the process. Thanks, Jialuo
