jgit performance update

[Shawn Pearce <spearce@spearce.org>]

With the help of Robin Rosenberg I've been able to make jgit's log
operation run (on average) within a few milliseconds of core Git.

Walking the 50,000 most recent commits from the Mozilla trunk[1]:

  $ time git rev-list --max-count=50000 HEAD >/dev/null

  core Git:  1.882s (average)
  jgit:      1.932s (average)

  (times are with hot cache and from repeated executions)

I think that is actually pretty good given that jgit is written
in Java using a fairly object-oriented design and has to deal with
some of the limitations of the language.

One of the biggest annoyances has been the fact that although Java
1.4 offers a way to mmap a file into the process, the overhead to
access that data seems to be far higher than just reading the file
content into a very large byte array, especially if we are going
to access that file content multiple times.  So jgit performs worse
than core Git early on while it copies everything from the OS buffer
cache into the Java process, but then performs reasonably well once
the internal cache is hot.  On the other hand using the mmap call
reduces early latency but hurts the access times so much that we're
talking closer to 3s average read times for the same log operation.

Anyway, jgit is now hopefully fast enough that we can start to build
some real functionality on top of it, and not need to wait several
minutes for answers from those features while debugging them.  :)


**1** This is the pack file from Jon Smirl's import attempt.

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