[PATCH 0/6] reuse on-disk deltas for fetches with bitmaps

This series more aggressively reuses on-disk deltas to serve fetches when reachability bitmaps tell us a more complete picture of what the client has. That saves server CPU and results in smaller packs. See the final patch for numbers and more discussion.

It's a resurrection of this very old series:

  https://public-inbox.org/git/20140326072215.GA31739@sigill.intra.peff.net/

The core idea is good, but it got left as "I should dig into this more to see if we can do even better". In fact, I _did_ do some of that digging, as you can see in the thread, so I'm mildly embarrassed not to have reposted it before now.

We've been using that original at GitHub since 2014, which I think helps demonstrate the correctness of the approach (and the numbers here and in that thread show that performance is generally a net win over the status quo).

I's rebased on top of the current master, since the original made some assumptions about struct object_entry that are no longer true post-v2.18 (due to the struct-shrinking exercise). So I fixed that and a few other rough edges. But that also means you're not getting code with 4-years of production testing behind it. :)

The other really ugly thing in the original was the way it leaked object_entry structs (though in practice that didn't really matter since we needed them until the end of the program anyway). This version fixes that.

  [1/6]: t/perf: factor boilerplate out of test_perf
  [2/6]: t/perf: factor out percent calculations
  [3/6]: t/perf: add infrastructure for measuring sizes
  [4/6]: t/perf: add perf tests for fetches from a bitmapped server
  [5/6]: pack-bitmap: save "have" bitmap from walk
  [6/6]: pack-objects: reuse on-disk deltas for thin "have" objects

 builtin/pack-objects.c             | 28 +++++++----
 pack-bitmap.c                      | 23 +++++++++-
 pack-bitmap.h                      |  7 +++
 pack-objects.c                     | 19 ++++++++
 pack-objects.h                     | 20 +++++++-
 t/perf/README                      | 25 ++++++++++
 t/perf/aggregate.perl              | 69 ++++++++++++++++++++++------
 t/perf/p5311-pack-bitmaps-fetch.sh | 45 ++++++++++++++++++
 t/perf/perf-lib.sh                 | 74 +++++++++++++++++++-----------
 9 files changed, 258 insertions(+), 52 deletions(-)
 create mode 100755 t/perf/p5311-pack-bitmaps-fetch.sh

-Peff