read-tree: use 'skip_cache_tree_update' option

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
d3da223f221 (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>

diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index f4cbe460b978447ba10bbb3e71b4faee1015d56d..45c6652444b6c12183cc43b38c3bc864b74e028b 100644 (file)
--- a/builtin/read-tree.c
+++ b/builtin/read-tree.c
@@ -249,6 +249,10 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
        if (opts.debug_unpack)
                opts.fn = debug_merge;
 
+       /* If we're going to prime_cache_tree later, skip cache tree update */
+       if (nr_trees == 1 && !opts.prefix)
+               opts.skip_cache_tree_update = 1;
+
        cache_tree_free(&active_cache_tree);
        for (i = 0; i < nr_trees; i++) {
                struct tree *tree = trees[i];

diff --git a/t/perf/p0006-read-tree-checkout.sh b/t/perf/p0006-read-tree-checkout.sh
index c481c012d2fc17a7944f308c2d6ef68acb284dee..325566e18ebc7fa7359d079935d6d7f81cd4dc54 100755 (executable)
--- a/t/perf/p0006-read-tree-checkout.sh
+++ b/t/perf/p0006-read-tree-checkout.sh
@@ -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

diff --git a/t/t1022-read-tree-partial-clone.sh b/t/t1022-read-tree-partial-clone.sh
index a9953b6a71c360a0c39d058ba61f9ff026ec04bf..da539716359614f6d91cd1db896b3981e1805d6d 100755 (executable)
--- a/t/t1022-read-tree-partial-clone.sh
+++ b/t/t1022-read-tree-partial-clone.sh
@@ -19,7 +19,7 @@ test_expect_success 'read-tree in partial clone prefetches in one batch' '
        git -C server config uploadpack.allowfilter 1 &&
        git -C server config uploadpack.allowanysha1inwant 1 &&
        git clone --bare --filter=blob:none "file://$(pwd)/server" client &&
-       GIT_TRACE_PACKET="$(pwd)/trace" git -C client read-tree $TREE &&
+       GIT_TRACE_PACKET="$(pwd)/trace" git -C client read-tree $TREE $TREE &&
 
        # "done" marks the end of negotiation (once per fetch). Expect that
        # only one fetch occurs.