Re: [PATCH v2 20/20] pack-revindex.c: avoid direct revindex access in 'offset_to_pack_pos()'

On Wed, Jan 13, 2021 at 10:42:29PM -0800, Junio C Hamano wrote:

Taylor Blau [off-list ref] writes:

quoted

To prepare for on-disk reverse indexes, remove a spot in
'offset_to_pack_pos()' that looks at the 'revindex' array in 'struct
packed_git'.

Hmph, I somehow would have expected that this clean-up would be done
before step [18/20], but that does not matter in the end.  The end
result looks fairly clean.

I wonder if the call overhead to pack_pos_to_offset(), relative to
the direct indexing of an in-core array revindex[] followed by an
access to a member .offset that we used to do, makes a measurable
difference in this tight loop, though.

I'm skeptical that it does (take that with a grain of salt, since I
haven't done any per-function tests with perf, only "how long does it
take to run 'git cat-file --batch-check=%(objectsize:disk)' and so on").

But even if it were to make a difference, it'll get dwarfed in the next
series by the time that we now _don't_ have to spend building and
sorting the reverse index in memory for each new process.

Thanks,
Taylor