Re: [PATCH] packfile: actually set approximate_object_count_valid

Jeff King [off-list ref] writes:

Subject: [PATCH] packfile: actually set approximate_object_count_valid

The approximate_object_count() function tries to compute the count only
once per process. But ever since it was introduced in 8e3f52d778
(find_unique_abbrev: move logic out of get_short_sha1(), 2016-10-03), we
failed to actually set the "valid" flag, meaning we'd compute it fresh
on every call.
...
  Test                            origin              HEAD
  ----------------------------------------------------------------------------
  5303.3: rev-list (1)            28.91(28.46+0.44)   29.03(28.65+0.38) +0.4%
  5303.4: abbrev-commit (1)       1.18(1.06+0.11)     1.17(1.02+0.14) -0.8%
  5303.7: rev-list (50)           28.95(28.56+0.38)   29.50(29.17+0.32) +1.9%
  5303.8: abbrev-commit (50)      3.67(3.56+0.10)     3.57(3.42+0.15) -2.7%
  5303.11: rev-list (1000)        30.34(29.89+0.43)   30.82(30.35+0.46) +1.6%
  5303.12: abbrev-commit (1000)   86.82(86.52+0.29)   77.82(77.59+0.22) -10.4%

Yuk, this is quite extreme.

  5303.15: load 10,000 packs      0.08(0.02+0.05)     0.08(0.02+0.06) +0.0%

It doesn't help at all when we have 1 pack (5303.4), but we get a 10%
speedup when there are 1000 packs (5303.12). That's a modest speedup for
a case that's already slow and we'd hope to avoid in general (note how
slow it is even after, because we have to look in each of those packs
for abbreviations). But it's a one-line change that clearly matches the
original intent, so it seems worth doing.

The included perf test may also be useful for keeping an eye on any
regressions in the overall abbreviation code.

Reported-by: Rasmus Villemoes <redacted>
Signed-off-by: Jeff King <redacted>
---
 packfile.c                 | 1 +
 t/perf/p5303-many-packs.sh | 4 ++++
 2 files changed, 5 insertions(+)

Thanks for finding and fixing.  I agree that this is worth doing.