On Fri, Mar 24, 2023 at 3:32 AM Jakub Narębski <jnareb@xxxxxxxxx> wrote: > > Hello Taylor, > > Thanks for a fast response. > > Taylor Blau <me@xxxxxxxxxxxx> writes: > > 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/pull.1357.git.1663609659.gitgitgadget@xxxxxxxxx/ > > > > 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. > > When I searched the mailing list archives, the thread was never continued. Hello community, I have to apologize for the fact that I didn't continue the patch series. I wasn't involved in the community either. I am currently too busy to enhance my skills to get into a company of "my dream engineering environment". The problem is that it needs much effort and time to achieve that. I have always had a love for the Git project and the community. But unfortunately I can't contribute to it right now and I don't think I can contribute to it prior to my course ends (i.e. June, 2023). I would be happy if anybody else pick the issue and continue the work where I left off. I am even ready to guide/mentor/help. There are certain things in my mind (other than roaring bitmaps) that I previously shared with Kaartik and Taylor. I will continue to be a part of this community and will make contributions after my college ends. > >> 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 ;-). > > Ah, so it is mostly the last issue - lack of a potential mentor for > conntinuing this project. > > > 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. Yeah, It is big enough to take an extra 3 months of code and discussions. > > 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? > > As far as I understand it, after reading articles about EWAH[2] and > about Roaring Bitmaps[3][4], the Roaring have the advantage that you > don't need to decompress (inflate) bitmaps to perform bitwise operations > on them. > > Run-Length-Encoding (RLE) formats like EWAH can be made to perform > operations without decompressing, but only if operations are symmetric. > The AND and OR operations are symmetrical, but AND NOT is not. The last > is used by Git to find "want"-ed that are not present (not "have") is > not. That is why Git needs to decompress bitmap and perform operation. > > If I understand it correctly, for both cases (EWAH and Roaring) you can > do membership check without decompressing bitmap. > > > [2] Daniel Lemire et al. "Sorting improves word-aligned bitmap indexes", > arXiv:0901.3751 > > [3] Samy Chambi, Daniel Lemire et al. "Optimizing Druid with Roaring > bitmaps", https://dl.acm.org/doi/10.1145/2938503.2938515 > [4] Daniel Lemire et al. "Roaring Bitmaps: Implementation of an > Optimized Software Library", arXiv:1709.07821v3 > > > > > 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. > > Well, if Roaring were to be significantly slower when deflating, but > only slightly faster when using / inflating, that would affect their > evaluation. IMHO, I don't think Roaring bitmaps would make any significant performance improvements. It may be faster to decompress, but I believe it takes more in memory computation than the EWAH. My biggest concern is its dynamic nature. It can dynamically change its underlying data structure into an array, bitmap or RLE. I didn't test the performance though and I shouldn't draw any conclusions about it. > > > > [1]: https://lore.kernel.org/git/CAPOJW5wkXrV8eOysz6aJ5jN2u_u-iTX_3om3tSDKw+EmfCJBEw@xxxxxxxxxxxxxx/ > > Regards, > -- > Jakub Narębski I am extremely sorry for the inconvenience and I hope you'll understand the situation. Thanks :) Regards, Abhradeep Chakraborty
