p5313: add size comparison test

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 <[email protected]>

Author: derrickstolee

t/perf/p5313-pack-objects.sh

@@ -38,6 +38,14 @@ test_size 'thin pack size with --path-walk' '
 	test_file_size 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' '
+	test_file_size out
+'
+
 test_perf 'big pack' '
 	git pack-objects --stdout --no-reuse-delta --revs \
 		--sparse <in-big >out
@@ -56,6 +64,14 @@ test_size 'big pack size with --path-walk' '
 	test_file_size 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' '
+	test_file_size out
+'
+
 test_perf 'repack' '
 	git repack -adf
 '
@@ -74,4 +90,13 @@ test_size 'repack size with --path-walk' '
 	test_file_size "$pack"
 '
 
+test_perf 'repack with --full-name-hash' '
+	git repack -adf --full-name-hash
+'
+
+test_size 'repack size with --full-name-hash' '
+	pack=$(ls .git/objects/pack/pack-*.pack) &&
+	test_file_size "$pack"
+'
+
 test_done