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

From: Jakub Narębski <jnareb@gmail.com>
Date: Aug 1, 2023, 11:34 UTC
Message-ID: <CANQwDwe8Po-2KxNjWQ+RW+hYGLF=4sYTjQJxcVSAOtbtpfVRhQ@mail.gmail.com>
In-Reply-To: <CAFQ2z_MzWauauzq_fKdcKTXahutLtADb7uHTh7ysinGNOx75nQ@mail.gmail.com>

Hello,

On Tue, 1 Aug 2023 at 13:26, Han-Wen Nienhuys <hanwen@google.com> wrote:

Show 23 quoted lines

> On Mon, Jul 31, 2023 at 10:18 PM Taylor Blau <me@ttaylorr.com> wrote:
> >
> > I haven't proved conclusively one way or the other where Roaring+Run is
> > significantly faster than EWAH or vice-versa. There are some cases where
> > the former is a clear winner, and other cases where it's the latter.
> >
> > In any event, my extremely WIP patches to make this mostly work are
> > available here:
> >
> >   https://github.com/ttaylorr/git/compare/tb/roaring-bitmaps
> >
>
> thanks. For anyone reading along, the changes to JGit are here
>
> https://git.eclipse.org/r/c/jgit/jgit/+/203448
>
> I was looking into this because I was hoping that roaring might
> decrease peak memory usage.
>
> I don't have firm evidence that it's better or worse, but I did
> observe that runtime and memory usage during GC (which is heavy on
> bitmap operations due to delta/xor encoding) was unchanged. That makes
> me pessimistic that there are significant gains to be had.

The major advantage Roaring bitmaps have over EWAH and other simple Run Length Encoding based compression algorithms is that bitmap operations can be done on compressed bitmaps: there is no need to uncompress bitmap to do (want1 OR want2 AND NOT have).

If I remember correctly, Git (the C implementation) basically un-compresses bitmaps to make use of them when using them during fetch.

Some operations can be done on EWAH without decompression, but non-symmetric full-bitmap operation line AND NOT is not one of them.

Best,

-- 
Jakub Narębski