unpack-trees: avoid duplicate ODB lookups during checkout
Teach traverse_trees_recursive() to not do redundant ODB lookups when both directories refer to the same OID. In operations such as read-tree and checkout, there will likely be many peer directories that have the same OID when the differences between the commits are relatively small. In these cases we can avoid hitting the ODB multiple times for the same OID. This patch handles n=2 and n=3 cases and simply copies the data rather than repeating the fill_tree_descriptor(). ================ On the Windows repo (500K trees, 3.1M files, 450MB index), this reduced the overall time by 0.75 seconds when cycling between 2 commits with a single file difference. (avg) before: 22.699 (avg) after: 21.955 =============== ================ On Linux using p0006-read-tree-checkout.sh with linux.git: Test HEAD^ HEAD ------------------------------------------------------------------------------------------------------- 0006.2: read-tree br_base br_ballast (57994) 0.24(0.20+0.03) 0.24(0.22+0.01) +0.0% 0006.3: switch between br_base br_ballast (57994) 10.58(6.23+2.86) 10.67(5.94+2.87) +0.9% 0006.4: switch between br_ballast br_ballast_plus_1 (57994) 0.60(0.44+0.17) 0.57(0.44+0.14) -5.0% 0006.5: switch between aliases (57994) 0.59(0.48+0.13) 0.57(0.44+0.15) -3.4% ================ Signed-off-by: NJeff Hostetler <jeffhost@microsoft.com> Signed-off-by: NJunio C Hamano <gitster@pobox.com>
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