From 911d14203c019d52431b1197dcbf44f163eac024 Mon Sep 17 00:00:00 2001 From: Jonathan Tan Date: Tue, 3 Dec 2024 13:52:54 -0800 Subject: [PATCH 1/3] index-pack --promisor: dedup before checking links Commit c08589efdc (index-pack: repack local links into promisor packs, 2024-11-01) fixed a bug with what was believed to be a negligible decrease in performance [1] [2]. But at $DAYJOB, with at least one repo, it was found that the decrease in performance was very significant. Looking at the patch, whenever we parse an object in the packfile to be indexed, we check the targets of all its outgoing links for its existence. However, this could be optimized by first collecting all such targets into an oidset (thus deduplicating them) before checking. Teach Git to do that. On a certain fetch from the aforementioned repo, this improved performance from approximately 7 hours to 24m47.815s. This number will be further reduced in a subsequent patch. [1] https://lore.kernel.org/git/CAG1j3zGiNMbri8rZNaF0w+yP+6OdMz0T8+8_Wgd1R_p1HzVasg@mail.gmail.com/ [2] https://lore.kernel.org/git/20241105212849.3759572-1-jonathantanmy@google.com/ Signed-off-by: Jonathan Tan Signed-off-by: Junio C Hamano --- builtin/index-pack.c | 75 +++++++++++++++++++++++++------------------- 1 file changed, 42 insertions(+), 33 deletions(-) diff --git a/builtin/index-pack.c b/builtin/index-pack.c index 95babdc5ea7e72..347a92c2f1d75e 100644 --- a/builtin/index-pack.c +++ b/builtin/index-pack.c @@ -155,11 +155,11 @@ static int input_fd, output_fd; static const char *curr_pack; /* - * local_links is guarded by read_mutex, and record_local_links is read-only in - * a thread. + * outgoing_links is guarded by read_mutex, and record_outgoing_links is + * read-only in a thread. */ -static struct oidset local_links = OIDSET_INIT; -static int record_local_links; +static struct oidset outgoing_links = OIDSET_INIT; +static int record_outgoing_links; static struct thread_local_data *thread_data; static int nr_dispatched; @@ -812,18 +812,12 @@ static int check_collison(struct object_entry *entry) return 0; } -static void record_if_local_object(const struct object_id *oid) +static void record_outgoing_link(const struct object_id *oid) { - struct object_info info = OBJECT_INFO_INIT; - if (oid_object_info_extended(the_repository, oid, &info, 0)) - /* Missing; assume it is a promisor object */ - return; - if (info.whence == OI_PACKED && info.u.packed.pack->pack_promisor) - return; - oidset_insert(&local_links, oid); + oidset_insert(&outgoing_links, oid); } -static void do_record_local_links(struct object *obj) +static void do_record_outgoing_links(struct object *obj) { if (obj->type == OBJ_TREE) { struct tree *tree = (struct tree *)obj; @@ -837,16 +831,16 @@ static void do_record_local_links(struct object *obj) */ return; while (tree_entry_gently(&desc, &entry)) - record_if_local_object(&entry.oid); + record_outgoing_link(&entry.oid); } else if (obj->type == OBJ_COMMIT) { struct commit *commit = (struct commit *) obj; struct commit_list *parents = commit->parents; for (; parents; parents = parents->next) - record_if_local_object(&parents->item->object.oid); + record_outgoing_link(&parents->item->object.oid); } else if (obj->type == OBJ_TAG) { struct tag *tag = (struct tag *) obj; - record_if_local_object(get_tagged_oid(tag)); + record_outgoing_link(get_tagged_oid(tag)); } } @@ -896,7 +890,7 @@ static void sha1_object(const void *data, struct object_entry *obj_entry, free(has_data); } - if (strict || do_fsck_object || record_local_links) { + if (strict || do_fsck_object || record_outgoing_links) { read_lock(); if (type == OBJ_BLOB) { struct blob *blob = lookup_blob(the_repository, oid); @@ -928,8 +922,8 @@ static void sha1_object(const void *data, struct object_entry *obj_entry, die(_("fsck error in packed object")); if (strict && fsck_walk(obj, NULL, &fsck_options)) die(_("Not all child objects of %s are reachable"), oid_to_hex(&obj->oid)); - if (record_local_links) - do_record_local_links(obj); + if (record_outgoing_links) + do_record_outgoing_links(obj); if (obj->type == OBJ_TREE) { struct tree *item = (struct tree *) obj; @@ -1779,28 +1773,43 @@ static void repack_local_links(void) struct strbuf line = STRBUF_INIT; struct oidset_iter iter; struct object_id *oid; - char *base_name; + char *base_name = NULL; - if (!oidset_size(&local_links)) + if (!oidset_size(&outgoing_links)) return; - base_name = mkpathdup("%s/pack/pack", repo_get_object_directory(the_repository)); + oidset_iter_init(&outgoing_links, &iter); + while ((oid = oidset_iter_next(&iter))) { + struct object_info info = OBJECT_INFO_INIT; + if (oid_object_info_extended(the_repository, oid, &info, 0)) + /* Missing; assume it is a promisor object */ + continue; + if (info.whence == OI_PACKED && info.u.packed.pack->pack_promisor) + continue; - strvec_push(&cmd.args, "pack-objects"); - strvec_push(&cmd.args, "--exclude-promisor-objects-best-effort"); - strvec_push(&cmd.args, base_name); - cmd.git_cmd = 1; - cmd.in = -1; - cmd.out = -1; - if (start_command(&cmd)) - die(_("could not start pack-objects to repack local links")); + if (!cmd.args.nr) { + base_name = mkpathdup( + "%s/pack/pack", + repo_get_object_directory(the_repository)); + strvec_push(&cmd.args, "pack-objects"); + strvec_push(&cmd.args, + "--exclude-promisor-objects-best-effort"); + strvec_push(&cmd.args, base_name); + cmd.git_cmd = 1; + cmd.in = -1; + cmd.out = -1; + if (start_command(&cmd)) + die(_("could not start pack-objects to repack local links")); + } - oidset_iter_init(&local_links, &iter); - while ((oid = oidset_iter_next(&iter))) { if (write_in_full(cmd.in, oid_to_hex(oid), the_hash_algo->hexsz) < 0 || write_in_full(cmd.in, "\n", 1) < 0) die(_("failed to feed local object to pack-objects")); } + + if (!cmd.args.nr) + return; + close(cmd.in); out = xfdopen(cmd.out, "r"); @@ -1899,7 +1908,7 @@ int cmd_index_pack(int argc, } else if (skip_to_optional_arg(arg, "--keep", &keep_msg)) { ; /* nothing to do */ } else if (skip_to_optional_arg(arg, "--promisor", &promisor_msg)) { - record_local_links = 1; + record_outgoing_links = 1; } else if (starts_with(arg, "--threads=")) { char *end; nr_threads = strtoul(arg+10, &end, 0); From 36198026d85848119a8eeb9286d10e795a9e0461 Mon Sep 17 00:00:00 2001 From: Jonathan Tan Date: Tue, 3 Dec 2024 13:52:55 -0800 Subject: [PATCH 2/3] index-pack --promisor: don't check blobs As a follow-up to the parent of this commit, it was found that not checking for the existence of blobs linked from trees sped up the fetch from 24m47.815s to 2m2.127s. Teach Git to do that. The tradeoff of not checking blobs is documented in a code comment. (Blobs may also be linked from tag objects, but it is impossible to know the type of an object linked from a tag object without looking it up in the object database, so the code for that is untouched.) Signed-off-by: Jonathan Tan Signed-off-by: Junio C Hamano --- builtin/index-pack.c | 31 ++++++++++++++++++++++++++++++- 1 file changed, 30 insertions(+), 1 deletion(-) diff --git a/builtin/index-pack.c b/builtin/index-pack.c index 347a92c2f1d75e..f811d1112a302a 100644 --- a/builtin/index-pack.c +++ b/builtin/index-pack.c @@ -817,6 +817,35 @@ static void record_outgoing_link(const struct object_id *oid) oidset_insert(&outgoing_links, oid); } +static void maybe_record_name_entry(const struct name_entry *entry) +{ + /* + * Checking only trees here results in a significantly faster packfile + * indexing, but the drawback is that if the packfile to be indexed + * references a local blob only directly (that is, never through a + * local tree), that local blob is in danger of being garbage + * collected. Such a situation may arise if we push local commits, + * including one with a change to a blob in the root tree, and then the + * server incorporates them into its main branch through a "rebase" or + * "squash" merge strategy, and then we fetch the new main branch from + * the server. + * + * This situation has not been observed yet - we have only noticed + * missing commits, not missing trees or blobs. (In fact, if it were + * believed that only missing commits are problematic, one could argue + * that we should also exclude trees during the outgoing link check; + * but it is safer to include them.) + * + * Due to the rarity of the situation (it has not been observed to + * happen in real life), and because the "penalty" in such a situation + * is merely to refetch the missing blob when it's needed (and this + * happens only once - when refetched, the blob goes into a promisor + * pack, so it won't be GC-ed, the tradeoff seems worth it. + */ + if (S_ISDIR(entry->mode)) + record_outgoing_link(&entry->oid); +} + static void do_record_outgoing_links(struct object *obj) { if (obj->type == OBJ_TREE) { @@ -831,7 +860,7 @@ static void do_record_outgoing_links(struct object *obj) */ return; while (tree_entry_gently(&desc, &entry)) - record_outgoing_link(&entry.oid); + maybe_record_name_entry(&entry); } else if (obj->type == OBJ_COMMIT) { struct commit *commit = (struct commit *) obj; struct commit_list *parents = commit->parents; From 1a14c857dbac02715725e8abcff35266c4c5ac93 Mon Sep 17 00:00:00 2001 From: Jonathan Tan Date: Tue, 3 Dec 2024 13:52:56 -0800 Subject: [PATCH 3/3] index-pack --promisor: also check commits' trees Commit c08589efdc (index-pack: repack local links into promisor packs, 2024-11-01) seems to contain an oversight in that the tree of a commit is not checked. Teach git to check these trees. The fix slows down a fetch from a certain repo at $DAYJOB from 2m2.127s to 2m45.052s, but in order to make the fetch correct, it seems worth it. In order to test this, we could create server and client repos as follows... C S \ / O (O and C are commits both on the client and server. S is a commit only on the server. C and S have the same tree but different commit messages. The diff between O and C is non-zero.) ...and then, from the client, fetch S from the server. In theory, the client declares "have C" and the server can use this information to exclude S's tree (since it knows that the client has C's tree, which is the same as S's tree). However, it is also possible for the server to compute that it needs to send S and not O, and proceed from there; therefore the objects of C are not considered at all when determining what to send in the packfile. In order to prevent a test of client functionality from having such a dependence on server behavior, I have not included such a test. Signed-off-by: Jonathan Tan Signed-off-by: Junio C Hamano --- builtin/index-pack.c | 1 + 1 file changed, 1 insertion(+) diff --git a/builtin/index-pack.c b/builtin/index-pack.c index f811d1112a302a..8e600a58bf1b44 100644 --- a/builtin/index-pack.c +++ b/builtin/index-pack.c @@ -865,6 +865,7 @@ static void do_record_outgoing_links(struct object *obj) struct commit *commit = (struct commit *) obj; struct commit_list *parents = commit->parents; + record_outgoing_link(get_commit_tree_oid(commit)); for (; parents; parents = parents->next) record_outgoing_link(&parents->item->object.oid); } else if (obj->type == OBJ_TAG) {