Re: [PATCH] packfile: actually set approximate_object_count_valid

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> 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 <rv@rasmusvillemoes.dk>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  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.