[PATCH 1/4] pack-bitmap: write lookup table extension by default

The lookup table extension for reachability bitmaps was first introduced via 3fe0121479 (Merge branch 'ac/bitmap-lookup-table', 2022-09-05). For each bitmapped commit, the lookup table encodes three unsigned integers:

  - The pack position of the commit.
  - An offset within the *.bitmap itself where that commit's bitmap
    lives.
  - An index into the same table where information on that commit's XOR
    pair can be found.

Lookup tables make bitmap operations faster because they no longer have to read the entire *.bitmap file to discover what commits have corresponding reachability bitmaps.

When bitmap lookup tables were first introduced, we established a baseline level of performance in p5310 with and without lookup tables. Here is the baseline without:

    Test                                                    this tree
    -----------------------------------------------------------------------
    5310.6: simulated clone                               14.04(5.78+1.79)
    5310.7: simulated fetch                               1.95(3.05+0.20)
    5310.8: pack to file (bitmap)                         44.73(20.55+7.45)
    5310.9: rev-list (commits)                            0.78(0.46+0.10)
    5310.10: rev-list (objects)                           4.07(3.97+0.08)
    5310.11: rev-list with tag negated via --not          0.06(0.02+0.03)
             --all (objects)
    5310.12: rev-list with negative tag (objects)         0.21(0.15+0.05)
    5310.13: rev-list count with blob:none                0.24(0.17+0.06)
    5310.14: rev-list count with blob:limit=1k            7.07(5.92+0.48)
    5310.15: rev-list count with tree:0                   0.25(0.17+0.07)
    5310.16: simulated partial clone                      5.67(3.28+0.64)
    5310.18: clone (partial bitmap)                       16.05(8.34+1.86)
    5310.19: pack to file (partial bitmap)                59.76(27.22+7.43)
    5310.20: rev-list with tree filter (partial bitmap)   0.90(0.18+0.16)

, and here is the same set of tests, this time with the lookup table enabled:

    Test                                                    this tree
    -----------------------------------------------------------------------
    5310.26: simulated clone                              13.69(5.72+1.78)
    5310.27: simulated fetch                              1.84(3.02+0.16)
    5310.28: pack to file (bitmap)                        45.63(20.67+7.50)
    5310.29: rev-list (commits)                           0.56(0.39+0.8)
    5310.30: rev-list (objects)                           3.77(3.74+0.08)
    5310.31: rev-list with tag negated via --not          0.05(0.02+0.03)
             --all (objects)
    5310.32: rev-list with negative tag (objects)         0.21(0.15+0.05)
    5310.33: rev-list count with blob:none                0.23(0.17+0.05)
    5310.34: rev-list count with blob:limit=1k            6.65(5.72+0.40)
    5310.35: rev-list count with tree:0                   0.23(0.16+0.06)
    5310.36: simulated partial clone                      5.57(3.26+0.59)
    5310.38: clone (partial bitmap)                       15.89(8.39+1.84)
    5310.39: pack to file (partial bitmap)                58.32(27.55+7.47)
    5310.40: rev-list with tree filter (partial bitmap)   0.73(0.18+0.15)

(All numbers here come from a ~2022-era copy of the kernel, via Abhradeep Chakraborty who implemented the lookup table extension).

In the almost three years since lookup tables were introduced, GitHub has used them in production without issue, taking advantage of the above performance benefits along the way.

Since this feature has had sufficient time to flush out any bugs and/or performance regressions, let's enable it by default so that all bitmap users can reap similar performance benefits.

[1]: https://lore.kernel.org/git/pull.1266.git.1655728395.gitgitgadget@gmail.com/

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/config/pack.adoc | 2 +-
 builtin/multi-pack-index.c     | 1 +
 builtin/pack-objects.c         | 2 +-
 3 files changed, 3 insertions(+), 2 deletions(-)