Re: [PATCH/RFC 0/6] reuse deltas found by bitmaps

Jeff King [off-list ref] writes:

Just looking at the 128-day case again, using bitmaps increased our
server CPU time _and_ made a much bigger pack. This series not only
fixes the CPU time regression, but it also drops the server CPU time to
almost nothing. That's a nice improvement, and it makes perfect sense:
we are reusing on-disk deltas instead of on-the-fly computing deltas
against the preferred bases.

True.

I think we could still add the objects from the tip of the client's HAVE
list.

That should make the result at least per to the non-bitmap case,
right?

This patch would still be a CPU win on top of that, because it
would reduce the number of objects which need a delta search in the
first place.

Yes.

So I think the next steps are probably:

  1. Measure the "all objects are preferred bases" approach and confirm
     that it is bad.

;-)

  2. Measure the "reused deltas become preferred bases" approach. I
     expect the resulting size to be slightly better than what I have
     now, but not as good as v1.9.0's size (but taking less CPU time).

Do you mean "the bases for reused deltas become preferred bases, so
that we can deltify more objects off of them"?

  3. Measure the "figure out boundaries and add them as preferred bases,
     like we do without bitmaps" approach. I expect this to behave
     similarly to v1.9.0.

Yes.

  4. Combine (2) and (3) and measure them. I'm _hoping_ this will give
     us the best of both worlds, but I still want to do the individual
     measurements so we can see where any improvement is coming from.

Sensible.  Thanks.