Currently for file-based repositories JGit will go over all refs in the
repository forach `ref-prefix` listed in the `ls-refs` command in git
protocol v2 request.
Native git, uses a different approach, where all refs are read once and
then for each ref, all `ref-prefix` filter values are checked in one
pass.
This change implements this approach in JGit only in the `RefDirectory`
backend. And makes `ref-prefix` filtering ~40% faster for repositories
with packed refs.
Different implementations were tested on a synthetic file repository
with 10k refs in `refs/heads/` and `290k` in `refs/changes`. Before
testing `git pack-refs` command was executed. All results are in
seconds.
Current Impl: 39.340 37.093 35.996
Nested for loops: 25.077 24.742 24.748
Nested streams: 24.827 24.890 27.525
Parallel stream + stream: 23.357 23.318 23.174
Nested parallel streams: 23.490 23.318 23.317
Stream + for loop: 23.147 23.210 23.126
Parallel stream + for loop: 23.317 23.423 22.847
The elapsed time was measured around `getRefByPrefix` call in
`Uploadapack.getFilteredRefs(Collection<String>)` (around lines 952 and
954). For testing a modified version of
`UploadPackTest.testV2LsRefsRefPrefix()` was used. The modifications
here included:
* shadowing protected `repo` variable with `FileRepository` pointing
to the synthetic repo with 300k refs described above,
* mimicking the git client clone request by adding `ref-prefix HEAD`,
`ref-prefix refs/heads/` and `ref-prefix refs/tags/`
Based on the above results, the implementation with parallel stream and
stream was selected.
Bug: 578550
Signed-off-by: Dariusz Luksza <dariusz.luksza@gmail.com>
Change-Id: I6416846c074b611ff6ec9d351dbafcfbcaf68e66
4f18c50950