Re: [PATCH 00/11] pack-bitmap: convert offset to ref deltas where possible

From: Jeff King <peff@peff.net>
Date: Oct 11, 2024, 08:01 UTC
Message-ID: <20241011080149.GA560372@coredump.intra.peff.net>
In-Reply-To: <20241011075451.GD18010@coredump.intra.peff.net>

On Fri, Oct 11, 2024 at 03:54:51AM -0400, Jeff King wrote:

Show 7 quoted lines

> [1] I wish we had good names to distinguish the various cases, because
>     the term "reuse" is kind of overloaded. The "slower" regular
>     object-sending path may still reuse verbatim bytes found in an
>     on-disk path. But this "blit out matching parts of a pack without
>     otherwise considering the objects" feature happens outside of that.
>     We called it "pack reuse" back in 2013, but that was not a good name
>     even then. I don't have a good suggestion, though.

Actually, confusing things more, there are really _three_ layers of reuse:

  1. At the beginning of a pack, we can blit out the bytes for objects
     starting from the beginning of the pack that are being sent (we
     know any delta will be satisfied since its base comes before it).

  2. After that, we process objects one by one, but do so very cheaply
     by just deciding if we can blit them out one by one, fixing up
     delta base offsets to account for gaps.

  3. Otherwise, we generate an object_entry struct in packing_data for
     them, try to find new deltas, and so on. We may then reuse the
     on-disk bytes after deciding they're suitable.

We call all of these "reuse", and certainly both (1) and (2) are "pack reuse", but I think that term is sufficiently vague that it could apply to (3) as well.

-Peff