Subject: Re: What is the status of GSoC 2022 work on making Git use roaring bitmaps?

From: Taylor Blau <[email-masked]>
Date: Thu, 23 Mar 2023 16:26:24 -0400

Hi Jakub,

On Thu, Mar 23, 2023 at 08:26:11PM +0100, Jakub Narębski wrote:
> Hello,
>
> Could you tell me what is the status of the Abhradeep Chakraborty work
> in integrating roaring bitmaps (using CRoaring) in addition to, or
> replacing current EWAH bitmaps (using ewok)? The last communication
> about this shows that the patches were on the road to being merged in,
> see e.g. https://medium.com/@abhra303/gsoc-final-report-feaaacfae737 ,
> but there is no mention of 'roaring' in Git's code or documentation.

Abhradeep started working on a prototype to teach Git how to read and
write Roaring+Run bitmaps in this series:

  https://lore.kernel.org/git/[email-masked]/

Some folks gave it a review, but there wasn't any serious traction and I
don't think that Abhradeep has had a chance to come back to the series.

For what it's worth, I would love if Abhradeep (or anybody else
interested in working on this area) picked it back up, either using that
series as a starting point or going from scratch.

> Moreover, there is no proposal to finish this on the GSoC 2023 ideas
> page: https://git.github.io/SoC-2023-Ideas/ .  Is it because it would be
> too small of a project?  Or maybe it turned out that roaring bitmaps
> were not a good idea - though I haven't found mentions of any benchmarks
> of roaring vs EWAH in the mailing list archives?  Or perhaps there is no
> one to mentor this proposal?

I don't have the capacity to mentor a student this cycle, and I am
probably the most interested among potential mentors in seeing this
project through ;-).

I don't think that it's too small (in fact, it was probably an error on
my part to include this as a potential stretch goal in Abhradeep's
project). We don't have any evidence that it's a good or bad idea.

Abhradeep promised[1] that he'd include some performance work in his
next version of that series. I think the main things we'd be interested
in are:

  - Does using Roaring provide a file-size advantage over
    EWAH-compressed bitmaps?
  - Does Roaring make it faster to inflate bitmaps? To deflate them?

Deflating bitmaps doesn't matter as much, IMHO, since that is a cost
that we pay only when we first have to compress bitmaps before writing
them. But if we could significantly reduce the inflation cost, that
would be an advantage to using Roaring+Run bitmaps over EWAH ones since
they would be faster to decompress at read-time.

Thanks,
Taylor

[1]: https://lore.kernel.org/git/[email-masked]/