t/perf: add performance test for sparse operations

Create a test script that takes the default performance test (the Git
codebase) and multiplies it by 256 using four layers of duplicated
trees of width four. This results in nearly one million blob entries in
the index. Then, we can clone this repository with sparse-checkout
patterns that demonstrate four copies of the initial repository. Each
clone will use a different index format or mode so peformance can be
tested across the different options.

Note that the initial repo is stripped of submodules before doing the
copies. This preserves the expected data shape of the sparse index,
because directories containing submodules are not collapsed to a sparse
directory entry.

Run a few Git commands on these clones, especially those that use the
index (status, add, commit).

Here are the results on my Linux machine:

Test
--------------------------------------------------------------
2000.2: git status (full-index-v3)             0.37(0.30+0.09)
2000.3: git status (full-index-v4)             0.39(0.32+0.10)
2000.4: git add -A (full-index-v3)             1.42(1.06+0.20)
2000.5: git add -A (full-index-v4)             1.26(0.98+0.16)
2000.6: git add . (full-index-v3)              1.40(1.04+0.18)
2000.7: git add . (full-index-v4)              1.26(0.98+0.17)
2000.8: git commit -a -m A (full-index-v3)     1.42(1.11+0.16)
2000.9: git commit -a -m A (full-index-v4)     1.33(1.08+0.16)

It is perhaps noteworthy that there is an improvement when using index
version 4. This is because the v3 index uses 108 MiB while the v4
index uses 80 MiB. Since the repeated portions of the directories are
very short (f3/f1/f2, for example) this ratio is less pronounced than in
similarly-sized real repositories.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/p2000-sparse-operations.sh

@@ -0,0 +1,84 @@
+#!/bin/sh
+
+test_description="test performance of Git operations using the index"
+
+. ./perf-lib.sh
+
+test_perf_default_repo
+
+SPARSE_CONE=f2/f4/f1
+
+test_expect_success 'setup repo and indexes' '
+	git reset --hard HEAD &&
+
+	# Remove submodules from the example repo, because our
+	# duplication of the entire repo creates an unlikely data shape.
+	if git config --file .gitmodules --get-regexp "submodule.*.path" >modules
+	then
+		git rm $(awk "{print \$2}" modules) &&
+		git commit -m "remove submodules" || return 1
+	fi &&
+
+	echo bogus >a &&
+	cp a b &&
+	git add a b &&
+	git commit -m "level 0" &&
+	BLOB=$(git rev-parse HEAD:a) &&
+	OLD_COMMIT=$(git rev-parse HEAD) &&
+	OLD_TREE=$(git rev-parse HEAD^{tree}) &&
+
+	for i in $(test_seq 1 4)
+	do
+		cat >in <<-EOF &&
+			100755 blob $BLOB	a
+			040000 tree $OLD_TREE	f1
+			040000 tree $OLD_TREE	f2
+			040000 tree $OLD_TREE	f3
+			040000 tree $OLD_TREE	f4
+		EOF
+		NEW_TREE=$(git mktree <in) &&
+		NEW_COMMIT=$(git commit-tree $NEW_TREE -p $OLD_COMMIT -m "level $i") &&
+		OLD_TREE=$NEW_TREE &&
+		OLD_COMMIT=$NEW_COMMIT || return 1
+	done &&
+
+	git sparse-checkout init --cone &&
+	git branch -f wide $OLD_COMMIT &&
+	git -c core.sparseCheckoutCone=true clone --branch=wide --sparse . full-index-v3 &&
+	(
+		cd full-index-v3 &&
+		git sparse-checkout init --cone &&
+		git sparse-checkout set $SPARSE_CONE &&
+		git config index.version 3 &&
+		git update-index --index-version=3
+	) &&
+	git -c core.sparseCheckoutCone=true clone --branch=wide --sparse . full-index-v4 &&
+	(
+		cd full-index-v4 &&
+		git sparse-checkout init --cone &&
+		git sparse-checkout set $SPARSE_CONE &&
+		git config index.version 4 &&
+		git update-index --index-version=4
+	)
+'
+
+test_perf_on_all () {
+	command="$@"
+	for repo in full-index-v3 full-index-v4
+	do
+		test_perf "$command ($repo)" "
+			(
+				cd $repo &&
+				echo >>$SPARSE_CONE/a &&
+				$command
+			)
+		"
+	done
+}
+
+test_perf_on_all git status
+test_perf_on_all git add -A
+test_perf_on_all git add .
+test_perf_on_all git commit -a -m A
+
+test_done