commit-graph write: use pack order when finding commits
Slightly optimize the "commit-graph write" step by using FOR_EACH_OBJECT_PACK_ORDER with for_each_object_in_pack(). See commit [1] and [2] for the facility and a similar optimization for "cat-file". On Linux it is around 5% slower to run: echo 1 >/proc/sys/vm/drop_caches && cat .git/objects/pack/* >/dev/null && git cat-file --batch-all-objects --batch-check --unordered Than the same thing with the "cat" omitted. This is as expected, since we're iterating in pack order and the "cat" is extra work. Before this change the opposite was true of "commit-graph write". We were 6% faster if we first ran "cat" to efficiently populate the FS cache for our sole big pack on linux.git, than if we had populated it via for_each_object_in_pack(). Now we're 3% faster without the "cat" instead. My tests were done on an unloaded Linux 3.10 system with 10 runs for each. Derrick Stolee did his own tests on Windows[3] showing a 2% improvement with a high degree of accuracy. 1. 736eb88f ("for_each_packed_object: support iterating in pack-order", 2018-08-10) 2. 0750bb5b ("cat-file: support "unordered" output for --batch-all-objects", 2018-08-10) 3. https://public-inbox.org/git/f71fa868-25e8-a9c9-46a6-611b987f1a8f@gmail.com/Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: NJunio C Hamano <gitster@pobox.com>
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