[PATCH v2 4/4] midx-write.c: use `--stdin-packs` when repacking

From: Taylor Blau <me@ttaylorr.com>
Date: Apr 1, 2024, 21:16 UTC
Message-ID: <b5d6ba5802aef6ddf1542f1b0efffe43c22436ba.1712006190.git.me@ttaylorr.com>
In-Reply-To: <cover.1712006190.git.me@ttaylorr.com>

When constructing a new pack `git multi-pack-index repack` provides a list of objects which is the union of objects in all MIDX'd packs which were "included" in the repack.

Though correct, this typically yields a poorly structured pack, since providing the objects list over stdin does not give pack-objects a chance to discover the namehash values for each object, leading to sub-optimal delta selection.

We can use `--stdin-packs` instead, which has a couple of benefits:

  - it does a supplemental walk over objects in the supplied list of
    packs to discover their namehash, leading to higher-quality delta
    selection

  - it requires us to list far less data over stdin; instead of listing
    each object in the resulting pack, we need only list the
    constituent packs from which those objects were selected in the MIDX

Of course, this comes at a slight cost: though we save time on listing packs versus objects over stdin[^1] (around ~650 milliseconds), we add a non-trivial amount of time walking over the given objects in order to find better deltas.

In general, this is likely to more closely match the user's expectations (i.e. that packs generated via `git multi-pack-index repack` are written with high-quality deltas). But if not, we can always introduce a new option in pack-objects to disable the supplemental object walk, which would yield a pure CPU-time savings, at the cost of the on-disk size of the resulting pack.

[^1]: In a patched version of Git that doesn't perform the supplemental
  object walk in `pack-objects --stdin-packs`, we save around ~650ms
  (from 5.968 to 5.325 seconds) when running `git multi-pack-index
  repack --batch-size=0` on git.git with all objects packed, and all
  packs in a MIDX.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx-write.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/midx-write.c b/midx-write.c
index 960cc46250..65e69d2de7 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1474,7 +1474,8 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
 	repo_config_get_bool(r, "repack.usedeltabaseoffset", &delta_base_offset);
 	repo_config_get_bool(r, "repack.usedeltaislands", &use_delta_islands);
 
-	strvec_push(&cmd.args, "pack-objects");
+	strvec_pushl(&cmd.args, "pack-objects", "--stdin-packs", "--non-empty",
+		     NULL);
 
 	strvec_pushf(&cmd.args, "%s/pack/pack", object_dir);
 
@@ -1498,16 +1499,15 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
 	}
 
 	cmd_in = xfdopen(cmd.in, "w");
-
-	for (i = 0; i < m->num_objects; i++) {
-		struct object_id oid;
-		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
-
-		if (!include_pack[pack_int_id])
+	for (i = 0; i < m->num_packs; i++) {
+		struct packed_git *p = m->packs[i];
+		if (!p)
 			continue;
 
-		nth_midxed_object_oid(&oid, m, i);
-		fprintf(cmd_in, "%s\n", oid_to_hex(&oid));
+		if (include_pack[i])
+			fprintf(cmd_in, "%s\n", pack_basename(p));
+		else
+			fprintf(cmd_in, "^%s\n", pack_basename(p));
 	}
 	fclose(cmd_in);

-- 
2.44.0.330.g158d2a670b4