[PATCH 3/4] p5313: add size comparison test

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Sep 9, 2024, 13:56 UTC
Message-ID: <ed3ea4281a909edac8623734283ef9c87548ec62.1725890211.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.1785.git.1725890210.gitgitgadget@gmail.com>

From: Derrick Stolee <stolee@gmail.com>

As custom options are added to 'git pack-objects' and 'git repack' to adjust how compression is done, use this new performance test script to demonstrate their effectiveness in performance and size.

The recently-added --full-name-hash option swaps the default name-hash algorithm with one that attempts to uniformly distribute the hashes based on the full path name instead of the last 16 characters.

This has a dramatic effect on full repacks for repositories with many versions of most paths. It can have a negative impact on cases such as pushing a single change.

This can be seen by running pt5313 on the open source fluentui repository [1]. Most commits will have this kind of output for the thin and big pack cases, though certain commits (such as [2]) will have problematic thin pack size for other reasons.

[1] https://github.com/microsoft/fluentui [2] a637a06df05360ce5ff21420803f64608226a875

Checked out at the parent of [2], I see the following statistics:

Test this tree ------------------------------------------------------------------ 5313.2: thin pack 0.02(0.01+0.01) 5313.3: thin pack size 1.1K 5313.4: thin pack with --full-name-hash 0.02(0.01+0.00) 5313.5: thin pack size with --full-name-hash 3.0K 5313.6: big pack 1.65(3.35+0.24) 5313.7: big pack size 58.0M 5313.8: big pack with --full-name-hash 1.53(2.52+0.18) 5313.9: big pack size with --full-name-hash 57.6M 5313.10: repack 176.52(706.60+3.53) 5313.11: repack size 446.7K 5313.12: repack with --full-name-hash 37.47(134.18+3.06) 5313.13: repack size with --full-name-hash 183.1K

Note that this demonstrates a 3x size _increase_ in the case that simulates a small "git push". The size change is neutral on the case of pushing the difference between HEAD and HEAD~1000.

However, the full repack case is both faster and more efficient.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
---
 t/perf/p5313-pack-objects.sh | 71 ++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100755 t/perf/p5313-pack-objects.sh

diff --git a/t/perf/p5313-pack-objects.sh b/t/perf/p5313-pack-objects.sh
new file mode 100755
index 00000000000..88ea0a4698a
--- /dev/null
+++ b/t/perf/p5313-pack-objects.sh
@@ -0,0 +1,71 @@
+#!/bin/sh
+
+test_description='Tests pack performance using bitmaps'
+. ./perf-lib.sh
+
+GIT_TEST_PASSING_SANITIZE_LEAK=0
+export GIT_TEST_PASSING_SANITIZE_LEAK
+
+test_perf_large_repo
+
+test_expect_success 'create rev input' '
+	cat >in-thin <<-EOF &&
+	$(git rev-parse HEAD)
+	^$(git rev-parse HEAD~1)
+	EOF
+
+	cat >in-big <<-EOF
+	$(git rev-parse HEAD)
+	^$(git rev-parse HEAD~1000)
+	EOF
+'
+
+test_perf 'thin pack' '
+	git pack-objects --thin --stdout --revs --sparse  <in-thin >out
+'
+
+test_size 'thin pack size' '
+	wc -c <out
+'
+
+test_perf 'thin pack with --full-name-hash' '
+	git pack-objects --thin --stdout --revs --sparse --full-name-hash <in-thin >out
+'
+
+test_size 'thin pack size with --full-name-hash' '
+	wc -c <out
+'
+
+test_perf 'big pack' '
+	git pack-objects --stdout --revs --sparse  <in-big >out
+'
+
+test_size 'big pack size' '
+	wc -c <out
+'
+
+test_perf 'big pack with --full-name-hash' '
+	git pack-objects --stdout --revs --sparse --full-name-hash <in-big >out
+'
+
+test_size 'big pack size with --full-name-hash' '
+	wc -c <out
+'
+
+test_perf 'repack' '
+	git repack -adf
+'
+
+test_size 'repack size' '
+	du -a .git/objects/pack | sort -nr | awk "{ print \$1; }" | head -n 1
+'
+
+test_perf 'repack with --full-name-hash' '
+	git repack -adf --full-name-hash
+'
+
+test_size 'repack size with --full-name-hash' '
+	du -a .git/objects/pack | sort -nr | awk "{ print \$1; }" | head -n 1
+'
+
+test_done

-- 
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