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.adoc

@@ -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] [--name-hash-version=<n>]
+	[--write-midx] [--name-hash-version=<n>] [--path-walk]
 
 DESCRIPTION
 -----------
@@ -258,6 +258,18 @@ linkgit:git-multi-pack-index[1]).
 	Provide this argument to the underlying `git pack-objects` process.
 	See linkgit:git-pack-objects[1] for full details.
 
+--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

@@ -43,7 +43,7 @@ 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] [--name-hash-version=<n>]"),
+	   "[--write-midx] [--name-hash-version=<n>] [--path-walk]"),
 	NULL
 };
 
@@ -63,6 +63,7 @@ struct pack_objects_args {
 	int quiet;
 	int local;
 	int name_hash_version;
+	int path_walk;
 	struct list_objects_filter_options filter_options;
 };
 
@@ -313,6 +314,8 @@ static void prepare_pack_objects(struct child_process *cmd,
 		strvec_pushf(&cmd->args, "--no-reuse-object");
 	if (args->name_hash_version)
 		strvec_pushf(&cmd->args, "--name-hash-version=%d", args->name_hash_version);
+	if (args->path_walk)
+		strvec_pushf(&cmd->args, "--path-walk");
 	if (args->local)
 		strvec_push(&cmd->args,  "--local");
 	if (args->quiet)
@@ -1184,6 +1187,8 @@ int cmd_repack(int argc,
 				N_("pass --no-reuse-object to git-pack-objects")),
 		OPT_INTEGER(0, "name-hash-version", &po_args.name_hash_version,
 				N_("specify the name hash version to use for grouping similar objects by path")),
+		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

@@ -64,4 +64,28 @@ do
 	'
 done
 
+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 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 with --path-walk' '
+	git repack -adf --path-walk
+'
+
+test_size 'repack size with --path-walk' '
+	wc -c <.git/objects/pack/pack-*.pack
+'
+
 test_done