reftable: fix perf regression when reading blocks of unwanted type

In fd888311fbc (reftable/table: move reading block into block reader,
2025-04-07), we have refactored how reftable blocks are read so that
most of the logic is contained in the "block.c" subsystem itself. Most
importantly, the whole logic to read the data itself is now contained in
that subsystem.

This change caused a significant performance regression though when
reading blocks that aren't of the specific type one is searching for:

    Benchmark 1: update-ref: create 100k refs (revision = fd888311fbc~)
      Time (mean ± σ):      2.171 s ±  0.028 s    [User: 1.189 s, System: 0.977 s]
      Range (min … max):    2.117 s …  2.206 s    10 runs

    Benchmark 2: update-ref: create 100k refs (revision = fd888311fbc)
      Time (mean ± σ):      3.418 s ±  0.030 s    [User: 2.371 s, System: 1.037 s]
      Range (min … max):    3.377 s …  3.473 s    10 runs

    Summary
      update-ref: create 100k refs (revision = fd888311fbc~) ran
        1.57 ± 0.02 times faster than update-ref: create 100k refs (revision = fd888311fbc)

The root caute of the performance regression is that we changed when
exactly blocks of an uninteresting type are being discarded. Previous to
the refactoring in the mentioned commit we'd load the block data, read
its type, notice that it's not the wanted type and discard the block.
After the commit though we don't discard the block immediately, but we
fully decode it only to realize that it's not the desired type. We then
discard the block again, but have already performed a bunch of pointless
work.

Fix the regression by making `reftable_block_init()` return early in
case the block is not of the desired type. This fixes the performance
hit:

    Benchmark 1: update-ref: create 100k refs (revision = HEAD~)
      Time (mean ± σ):      2.712 s ±  0.018 s    [User: 1.990 s, System: 0.716 s]
      Range (min … max):    2.682 s …  2.741 s    10 runs

    Benchmark 2: update-ref: create 100k refs (revision = HEAD)
      Time (mean ± σ):      1.670 s ±  0.012 s    [User: 0.991 s, System: 0.676 s]
      Range (min … max):    1.652 s …  1.693 s    10 runs

    Summary
      update-ref: create 100k refs (revision = HEAD) ran
        1.62 ± 0.02 times faster than update-ref: create 100k refs (revision = HEAD~)

Note that the baseline performance is lower than in the original due to
a couple of unrelated performance improvements that have landed since
the original commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions