repack: add --path-walk option

Since 'git pack-objects' supports a --path-walk option, allow passing it
through in 'git repack'. This presents interesting testing opportunities for
comparing the different repacking strategies against each other.

Add the --path-walk option to the performance tests in p5313.

For the microsoft/fluentui repo [1] checked out at a specific commit [2],
the results are very interesting:

Test                                           this tree
------------------------------------------------------------------
5313.2: thin pack                              0.40(0.47+0.04)
5313.3: thin pack size                                    1.2M
5313.4: thin pack with --full-name-hash        0.09(0.10+0.04)
5313.5: thin pack size with --full-name-hash             22.8K
5313.6: thin pack with --path-walk             0.08(0.06+0.02)
5313.7: thin pack size with --path-walk                  20.8K
5313.8: big pack                               2.16(8.43+0.23)
5313.9: big pack size                                    17.7M
5313.10: big pack with --full-name-hash        1.42(3.06+0.21)
5313.11: big pack size with --full-name-hash             18.0M
5313.12: big pack with --path-walk             2.21(8.39+0.24)
5313.13: big pack size with --path-walk                  17.8M
5313.14: repack                                98.05(662.37+2.64)
5313.15: repack size                                    449.1K
5313.16: repack with --full-name-hash          33.95(129.44+2.63)
5313.17: repack size with --full-name-hash              182.9K
5313.18: repack with --path-walk               106.21(121.58+0.82)
5313.19: repack size with --path-walk                   159.6K

[1] https://github.com/microsoft/fluentui
[2] e70848ebac1cd720875bccaa3026f4a9ed700e08

This repo suffers from having a lot of paths that collide in the name
hash, so examining them in groups by path leads to better deltas. Also,
in this case, the single-threaded implementation is competitive with the
full repack. This is saving time diffing files that have significant
differences from each other.

A similar, but private, repo has even more extremes in the thin packs:

Test                                           this tree
--------------------------------------------------------------
5313.2: thin pack                              2.39(2.91+0.10)
5313.3: thin pack size                                    4.5M
5313.4: thin pack with --full-name-hash        0.29(0.47+0.12)
5313.5: thin pack size with --full-name-hash             15.5K
5313.6: thin pack with --path-walk             0.35(0.31+0.04)
5313.7: thin pack size with --path-walk                  14.2K

Notice, however, that while the --full-name-hash version is working
quite well in these cases for the thin pack, it does poorly for some
other standard cases, such as this test on the Linux kernel repository:

Test                                           this tree
--------------------------------------------------------------
5313.2: thin pack                              0.01(0.00+0.00)
5313.3: thin pack size                                     310
5313.4: thin pack with --full-name-hash        0.00(0.00+0.00)
5313.5: thin pack size with --full-name-hash              1.4K
5313.6: thin pack with --path-walk             0.00(0.00+0.00)
5313.7: thin pack size with --path-walk                    310

Here, the --full-name-hash option does much worse than the default name
hash, but the path-walk option does exactly as well.

Signed-off-by: Derrick Stolee <[email protected]>

Author: derrickstolee

Documentation/git-repack.txt

@@ -11,7 +11,7 @@ SYNOPSIS
 [verse]
 'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]
 	[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]
-	[--write-midx] [--full-name-hash]
+	[--write-midx] [--full-name-hash] [--path-walk]
 
 DESCRIPTION
 -----------
@@ -251,6 +251,19 @@ linkgit:git-multi-pack-index[1]).
 	Write a multi-pack index (see linkgit:git-multi-pack-index[1])
 	containing the non-redundant packs.
 
+--path-walk::
+	This option passes the `--path-walk` option to the underlying
+	`git pack-options` process (see linkgit:git-pack-objects[1]).
+	By default, `git pack-objects` walks objects in an order that
+	presents trees and blobs in an order unrelated to the path they
+	appear relative to a commit's root tree. The `--path-walk` option
+	enables a different walking algorithm that organizes trees and
+	blobs by path. This has the potential to improve delta compression
+	especially in the presence of filenames that cause collisions in
+	Git's default name-hash algorithm. Due to changing how the objects
+	are walked, this option is not compatible with `--delta-islands`
+	or `--filter`.
+
 CONFIGURATION
 -------------
 

builtin/repack.c

@@ -41,7 +41,7 @@ static char *packdir, *packtmp_name, *packtmp;
 static const char *const git_repack_usage[] = {
 	N_("git repack [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]\n"
 	   "[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]\n"
-	   "[--write-midx] [--full-name-hash]"),
+	   "[--write-midx] [--full-name-hash] [--path-walk]"),
 	NULL
 };
 
@@ -61,6 +61,7 @@ struct pack_objects_args {
 	int quiet;
 	int local;
 	int full_name_hash;
+	int path_walk;
 	struct list_objects_filter_options filter_options;
 };
 
@@ -311,6 +312,8 @@ static void prepare_pack_objects(struct child_process *cmd,
 		strvec_pushf(&cmd->args, "--no-reuse-object");
 	if (args->full_name_hash)
 		strvec_pushf(&cmd->args, "--full-name-hash");
+	if (args->path_walk)
+		strvec_pushf(&cmd->args, "--path-walk");
 	if (args->local)
 		strvec_push(&cmd->args,  "--local");
 	if (args->quiet)
@@ -1210,6 +1213,8 @@ int cmd_repack(int argc,
 				N_("pass --no-reuse-object to git-pack-objects")),
 		OPT_BOOL(0, "full-name-hash", &po_args.full_name_hash,
 				N_("(EXPERIMENTAL!) pass --full-name-hash to git-pack-objects")),
+		OPT_BOOL(0, "path-walk", &po_args.path_walk,
+				N_("(EXPERIMENTAL!) pass --path-walk to git-pack-objects")),
 		OPT_NEGBIT('n', NULL, &run_update_server_info,
 				N_("do not run git-update-server-info"), 1),
 		OPT__QUIET(&po_args.quiet, N_("be quiet")),

t/perf/p5313-pack-objects.sh

@@ -36,6 +36,14 @@ test_size 'thin pack size with --full-name-hash' '
 	test_file_size out
 '
 
+test_perf 'thin pack with --path-walk' '
+	git pack-objects --thin --stdout --revs --sparse --path-walk <in-thin >out
+'
+
+test_size 'thin pack size with --path-walk' '
+	wc -c <out
+'
+
 test_perf 'big pack' '
 	git pack-objects --stdout --revs --sparse  <in-big >out
 '
@@ -52,6 +60,14 @@ test_size 'big pack size with --full-name-hash' '
 	test_file_size out
 '
 
+test_perf 'big pack with --path-walk' '
+	git pack-objects --stdout --revs --sparse --path-walk <in-big >out
+'
+
+test_size 'big pack size with --path-walk' '
+	wc -c <out
+'
+
 test_perf 'repack' '
 	git repack -adf
 '
@@ -70,4 +86,12 @@ test_size 'repack size with --full-name-hash' '
 	test_file_size "$pack"
 '
 
+test_perf 'repack with --path-walk' '
+	git repack -adf --path-walk
+'
+
+test_size 'repack size with --path-walk' '
+	wc -c <.git/objects/pack/pack-*.pack
+'
+
 test_done