Re: [PATCH 0/4] pack-objects: create new name-hash algorithm

From: Junio C Hamano <gitster@pobox.com>
Date: Sep 9, 2024, 18:06 UTC
Message-ID: <xmqqjzfkr9b0.fsf@gitster.g>
In-Reply-To: <pull.1785.git.1725890210.gitgitgadget@gmail.com>

"Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:

Show 6 quoted lines

> One way to find some improvement in these repositories is to increase the
> window size, which was an initial indicator that the delta compression could
> be improved, but was not a clear indicator. After some digging (and
> prototyping some analysis tools) the main discovery was that the current
> name-hash algorithm only considers the last 16 characters in the path name
> and has some naturally-occurring collisions within that scope.

Yes, as I explained in the other message, this "collision" is an integral part of the design to allow us gather candidates together that may yield good deltas among them. In addition, header files whose names end with ".h" tend to share a bit comment at the beginning of them in many projects, and the proximity (not "collision") of the hash value is used to make them delta candidates with each other.

I do agree that considering files at the same path from different (but close-by) revisions as the prime candidates is very important, but if you spread the "collissions" very thin by using "uniform distribution", I am afraid that you'd end up discarding anything but the blobs at the same path, which may go too far. Having name hash value that are close by no longer has any meaning in such a system.

I hope you can find a solution that strikes a good balance at the end of the series (I saw only the first step), but I suspect an easy way to avoid the downsides you observed is to use both. Compare with a handful of blobs taken from nearby commits (the original object order is roughly in traversal order, and you can take advantage of that fact) from exactly the same path (using your "uniform distribution") before comparing with the blobs with close value (of the current function) like the current implementation does, may go a long way.