Hello,

On Tue, 1 Aug 2023 at 13:26, Han-Wen Nienhuys [off-list ref] wrote:

On Mon, Jul 31, 2023 at 10:18 PM Taylor Blau [off-list ref] wrote:

quoted

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