[GSOC] Blog about weeks 4, 5

[Abhishek Kumar <abhishekkumar8222@gmail.com>]

Hello everyone!

Over the last two weeks, I have worked on refining the performance
report on generation numbers. Here are our conclusions:

- Corrected Commit Dates With Monotonically Offset (i.e.  generation
  number v5) performs better than topological levels but is still walks
  too many commits when compared with Corrected Commit Dates.

Number of commits walked (git merge-base v4.8 v4.9, on linux repository):

Topological Level                          : 635579
Corrected Commit Date                      : 167468
Corrected Commit Date With Monotonic Offset: 506577

As such, I am expecting that we will store Corrected Commit Date in an
additional chunk (called "generation data chunk") and store topological
levels into CDAT. Thus, old Git clients can operate as expected, with
new Git clients using the better generation number.

- Using a new chunk does affect the locality of reference but did not
  impact the performance appreciably.
- This does increase the size of commit graph file by nearly 5%.

You can read more in my report [1] and the pull request with
instructions to replicate the results [2].

[1]: https://lore.kernel.org/git/20200703082842.GA28027@Abhishek-Arch/T/#mda33f6e13873df55901768e8fd6d774282002146
[2]: https://github.com/abhishekkumar2718/git/pull/1

I talk a bit more about a patch I worked on, trying to improve
performance of commit graph write using buffers which ultimately did not
work and is dropped. Up next is actually implementing the generation
number and take care of all little details.

https://abhishekkumar2718.github.io/programming/2020/07/05/gsoc-weeks-4-5.html

Feedback and suggestions welcome!

Thanks
- Abhishek

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Re-sending this email as I forgot to cc git@vger.kernel.org