config: enable `pack.writeReverseIndex` by default

Back in e37d0b8730 (builtin/index-pack.c: write reverse indexes,
2021-01-25), Git learned how to read and write a pack's reverse index
from a file instead of in-memory.

A pack's reverse index is a mapping from pack position (that is, the
order that objects appear together in a ".pack")  to their position in
lexical order (that is, the order that objects are listed in an ".idx"
file).

Reverse indexes are consulted often during pack-objects, as well as
during auxiliary operations that require mapping between pack offsets,
pack order, and index index.

They are useful in GitHub's infrastructure, where we have seen a
dramatic increase in performance when writing ".rev" files[1]. In
particular:

  - an ~80% reduction in the time it takes to serve fetches on a popular
    repository, Homebrew/homebrew-core.

  - a ~60% reduction in the peak memory usage to serve fetches on that
    same repository.

  - a collective savings of ~35% in CPU time across all pack-objects
    invocations serving fetches across all repositories in a single
    datacenter.

Reverse indexes are also beneficial to end-users as well as forges. For
example, the time it takes to generate a pack containing the objects for
the 10 most recent commits in linux.git (representing a typical push) is
significantly faster when on-disk reverse indexes are available:

    $ { git rev-parse HEAD && printf '^' && git rev-parse HEAD~10 } >in
    $ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} pack-objects --delta-base-offset --revs --stdout <in >/dev/null'
    Benchmark 1: git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null
      Time (mean ± σ):     543.0 ms ±  20.3 ms    [User: 616.2 ms, System: 58.8 ms]
      Range (min … max):   521.0 ms … 577.9 ms    10 runs

    Benchmark 2: git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null
      Time (mean ± σ):     245.0 ms ±  11.4 ms    [User: 335.6 ms, System: 31.3 ms]
      Range (min … max):   226.0 ms … 259.6 ms    13 runs

    Summary
      'git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null' ran
	2.22 ± 0.13 times faster than 'git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null'

The same is true of writing a pack containing the objects for the 30
most-recent commits:

    $ { git rev-parse HEAD && printf '^' && git rev-parse HEAD~30 } >in
    $ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} pack-objects --delta-base-offset --revs --stdout <in >/dev/null'
    Benchmark 1: git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null
      Time (mean ± σ):     866.5 ms ±  16.2 ms    [User: 1414.5 ms, System: 97.0 ms]
      Range (min … max):   839.3 ms … 886.9 ms    10 runs

    Benchmark 2: git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null
      Time (mean ± σ):     581.6 ms ±  10.2 ms    [User: 1181.7 ms, System: 62.6 ms]
      Range (min … max):   567.5 ms … 599.3 ms    10 runs

    Summary
      'git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null' ran
	1.49 ± 0.04 times faster than 'git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null'

...and savings on trivial operations like computing the on-disk size of
a single (packed) object are even more dramatic:

    $ git rev-parse HEAD >in
    $ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --batch-check="%(objectsize:disk)" <in'
    Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in
      Time (mean ± σ):     305.8 ms ±  11.4 ms    [User: 264.2 ms, System: 41.4 ms]
      Range (min … max):   290.3 ms … 331.1 ms    10 runs

    Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in
      Time (mean ± σ):       4.0 ms ±   0.3 ms    [User: 1.7 ms, System: 2.3 ms]
      Range (min … max):     1.6 ms …   4.6 ms    1155 runs

    Summary
      'git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in' ran
       76.96 ± 6.25 times faster than 'git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in'

In the more than two years since e37d0b8730 was merged, Git's
implementation of on-disk reverse indexes has been thoroughly tested,
both from users enabling `pack.writeReverseIndexes`, and from GitHub's
deployment of the feature. The latter has been running without incident
for more than two years.

This patch changes Git's behavior to write on-disk reverse indexes by
default when indexing a pack, which should make the above operations
faster for everybody's Git installation after a repack.

(The previous commit explains some potential drawbacks of using on-disk
reverse indexes in certain limited circumstances, that essentially boil
down to a trade-off between time to generate, and time to access. For
those limited cases, the `pack.readReverseIndex` escape hatch can be
used).

[1]: https://github.blog/2021-04-29-scaling-monorepo-maintenance/#reverse-indexes

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/pack.txt

@@ -182,4 +182,4 @@ pack.writeReverseIndex::
 	linkgit:gitformat-pack[5])
 	for each new packfile that it writes in all places except for
 	linkgit:git-fast-import[1] and in the bulk checkin mechanism.
-	Defaults to false.
+	Defaults to true.

builtin/index-pack.c

@@ -1753,6 +1753,7 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	fsck_options.walk = mark_link;
 
 	reset_pack_idx_option(&opts);
+	opts.flags |= WRITE_REV;
 	git_config(git_index_pack_config, &opts);
 	if (prefix && chdir(prefix))
 		die(_("Cannot come back to cwd"));

builtin/pack-objects.c

@@ -4293,6 +4293,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	}
 
 	reset_pack_idx_option(&pack_idx_opts);
+	pack_idx_opts.flags |= WRITE_REV;
 	git_config(git_pack_config, NULL);
 	if (git_env_bool(GIT_TEST_WRITE_REV_INDEX, 0))
 		pack_idx_opts.flags |= WRITE_REV;

t/perf/p5312-pack-bitmaps-revs.sh

@@ -12,8 +12,7 @@ test_lookup_pack_bitmap () {
 	test_perf_large_repo
 
 	test_expect_success 'setup bitmap config' '
-		git config pack.writebitmaps true &&
-		git config pack.writeReverseIndex true
+		git config pack.writebitmaps true
 	'
 
 	# we need to create the tag up front such that it is covered by the repack and

t/t5325-reverse-index.sh

@@ -14,6 +14,7 @@ packdir=.git/objects/pack
 test_expect_success 'setup' '
 	test_commit base &&
 
+	test_config pack.writeReverseIndex false &&
 	pack=$(git pack-objects --all $packdir/pack) &&
 	rev=$packdir/pack-$pack.rev &&