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read-tree: use 'skip_cache_tree_update' option

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When running 'read-tree' with a single tree and no prefix,
'prime_cache_tree()' is called after the tree is unpacked. In that
situation, skip a redundant call to 'cache_tree_update()' in
'unpack_trees()' by enabling the 'skip_cache_tree_update' unpack option.

Removing the redundant cache tree update provides a substantial performance
improvement to 'git read-tree <tree-ish>', as shown by a test added to
'p0006-read-tree-checkout.sh':

Test                          before            after
----------------------------------------------------------------------
read-tree br_ballast_plus_1   3.94(1.80+1.57)   3.00(1.14+1.28) -23.9%

Note that the 'read-tree' in 't1022-read-tree-partial-clone.sh' is updated
to read two trees, rather than one. The test was first introduced in
d3da223f22 (cache-tree: prefetch in partial clone read-tree, 2021-07-23) to
exercise the 'cache_tree_update()' code path, as used in 'git merge'. Since
this patch drops the call to 'cache_tree_update()' in single-tree 'git
read-tree', change the test to use the two-tree variant so that
'cache_tree_update()' is called as intended.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
This commit is contained in:
Victoria Dye
2022-11-10 19:06:04 +00:00
committed by Taylor Blau
parent 0e47bca0f7
commit dc5d40f5bc
3 changed files with 13 additions and 1 deletions
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8
t/perf/p0006-read-tree-checkout.sh
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@ -49,6 +49,14 @@ test_perf "read-tree br_base br_ballast ($nr_files)" '
git read-tree -n -m br_base br_ballast
'
test_perf "read-tree br_ballast_plus_1 ($nr_files)" '
# Run read-tree 100 times for clearer performance results & comparisons
for i in $(test_seq 100)
do
git read-tree -n -m br_ballast_plus_1 || return 1
done
'
test_perf "switch between br_base br_ballast ($nr_files)" '
git checkout -q br_base &&
git checkout -q br_ballast