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

[Taylor Blau <me@ttaylorr.com>]

On Thu, Oct 10, 2024 at 01:20:06PM -0700, Junio C Hamano wrote:
> Taylor Blau <me@ttaylorr.com> writes:
>
> >> So when you pick the copy of Y out of another pack, what's so
> >> different?  After emitting Y to the resulting pack stream (and
> >> remembering where in the packstream you did so), when it is X's turn
> >> to be emitted, shouldn't you be able to compute the distance in the
> >> resulting packstream to represent X as an ofs-delta against Y, which
> >> should already be happening when you had both X and Y in the same
> >> original pack?
> >
> > Good question. The difference is that if you're reusing X and Y from
> > same pack, you know that Y occurs some number of bytes *before* X in the
> > resulting pack.
> >
> > But if Y comes from a different pack, it may get pushed further back in
> > the MIDX pseudo-pack order. So in that case the assembled pack may list
> > X before Y, in which case X cannot be an OFS_DELTA of Y, since offset
> > deltas require that the base object appears first.
>
> That is what we have always done even before we started bitmap based
> optimization.  If we happen to write Y before X, we consider doing
> ofs-delta for X, but otherwise we do ref-delta for X.  We do reorder
> fairly late in the pipeline when we notice that X that we are about
> to write out depends on Y that we haven't emitted to avoid this,
> though.  All of that the bitmap-based optimization code path should
> be able to imitate, I would think.

I see..., so yeah: before this patch series we would have not processed
X via the bitmap-powered pack reuse mechanism, and after this series we
will.

Thanks,
Taylor