[PATCH] midx.c: use `pack-objects --stdin-packs` when repacking
- From
Taylor Blau <me@ttaylorr.com>
- Date
- Sep 20, 2022, 02:08 UTC
- Message-ID
- <9195a9ecd11a19f2c7fb1c70136d2d13fa308010.1663639662.git.me@ttaylorr.com>
The `git multi-pack-index repack` command aggregates all objects in certain packs contained in the MIDX, largely dictated by the value of its `--batch-size` parameter.
To create the new pack, the MIDX machinery spawns a `pack-objects` sub-process, which it feeds the object IDs in each of the pack(s) that it wants to aggregate.
This implementation, which dates back to ce1e4a105b (midx: implement midx_repack(), 2019-06-10), predates the `--stdin-packs` mode in `pack-objects`. This mode (which was introduced a couple of years later in 339bce27f4 (builtin/pack-objects.c: add '--stdin-packs' option, 2021-02-22)) allows `pack-objects` callers to specify the contents of the output pack by indicating which packs to aggregate.
This patch replaces the pre-`--stdin-packs` invocation (where each object is given to `pack-objects` one by one) with the more modern `--stdin-packs` option.
This allows us to avoid some CPU cycles serializing and deserializing every object ID in all of the packs we're aggregating. It also avoids us having to send a potentially large amount of data down to `pack-objects`.
But more importantly, it generates slightly higher quality (read: more tightly compressed) packs, because of the reachability traversal that `--stdin-packs` does after the fact in order to gather namehash values which seed the delta selection process.
In practice, this seems to add a slight amount of overhead (on the order of a few seconds for git.git broken up into ~100 packs), in exchange for a modest reduction (on the order of ~3.5%) in the resulting pack size.
Signed-off-by: Taylor Blau <me@ttaylorr.com> --- Noticed this while working on a semi-related series in:
https://lore.kernel.org/git/cover.1663638929.git.me@ttaylorr.com/T/
the savings here are pretty modest, but this is in line with the strategy we use in the `--geometric` repack mode, which performs a similar task.
midx.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-)
diff --git a/midx.c b/midx.c index c27d0e5f15..c06f0f00e6 100644 --- a/midx.c +++ b/midx.c @@ -1973,6 +1973,7 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size, struct child_process cmd = CHILD_PROCESS_INIT; FILE *cmd_in; struct strbuf base_name = STRBUF_INIT; + struct strbuf scratch = STRBUF_INIT; struct multi_pack_index *m = lookup_multi_pack_index(r, object_dir); /* @@ -1997,7 +1998,7 @@ 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", NULL); strbuf_addstr(&base_name, object_dir); strbuf_addstr(&base_name, "/pack/pack"); @@ -2026,17 +2027,17 @@ 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); + for (i = 0; i < m->num_packs; i++) { + strbuf_reset(&scratch); - if (!include_pack[pack_int_id]) - continue; + strbuf_addstr(&scratch, m->pack_names[i]); + strbuf_strip_suffix(&scratch, ".idx"); + strbuf_addstr(&scratch, ".pack"); - nth_midxed_object_oid(&oid, m, i); - fprintf(cmd_in, "%s\n", oid_to_hex(&oid)); + fprintf(cmd_in, "%s%s\n", include_pack[i] ? "" : "^", scratch.buf); } fclose(cmd_in); + strbuf_release(&scratch); if (finish_command(&cmd)) { error(_("could not finish pack-objects")); -- 2.37.0.1.g1379af2e9d