[jgit-dev] RFC: Optimized "single-commit" push

[Justin Santa Barbara <justin@xxxxxxxxxxxx>]

I am using git as a versioned store in a project I'm working on, with
a fairly big repo, and using JGit as both the git client (which is
actually a RESTful server) and the git server (using
jgit.http.server).  Performance is generally good with frequent server
GCs and sufficient memory, but a push will sometimes take a long time
in the "counting objects" phase (30 seconds or more).

Because of my use-case though, the problem is constrained: I am
pushing a single commit to the remote server, and there are only a
handful of changed files (typically one).  I created an experimental
patch that detects this case and optimizes it by directly comparing
the new commit's tree to the base commit's tree:

https://github.com/justinsb/jgit/commit/9db165e88d162c7f052f6c58784c16d4cd830b3e

There are limitations however, which is why I gated it behind a
boolean option.  The biggest is that if the new files are already
available on the server on a different branch, we won't reuse them
(e.g. cherry-picks).

A few questions I would love some feedback on:

1. Is this something that might be considered for inclusion into jgit?
2. Should I instead figure out a way to expose the
PackWriter.preparePack(Iterator<RevObject>) method, perhaps by passing
a list containing the known set of objects when doing the push? I
imagine that would be more general and thus more welcome in jgit
(though obviously harder to use!)
3. Am I doing something obviously wrong to cause a slow 'counting
objects' phase (I expect it is just the repo size - it is currently
about 250k objects)

Many thanks,
Justin