From: Derrick Stolee <dstolee@xxxxxxxxxxxxx>
The multi-pack-index allows searching for objects across multiple
packs using one object list. The original design gains many of
these performance benefits by keeping the packs in the
multi-pack-index out of the packed_git list.
Unfortunately, this has one major drawback. If the multi-pack-index
covers thousands of packs, and a command loads many of those packs,
then we can hit the limit for open file descriptors. The
close_one_pack() method is used to limit this resource, but it
only looks at the packed_git list, and uses an LRU cache to prevent
thrashing.
Instead of complicating this close_one_pack() logic to include
direct references to the multi-pack-index, simply add the packs
opened by the multi-pack-index to the packed_git list. This
immediately solves the file-descriptor limit problem, but requires
some extra steps to avoid performance issues or other problems:
1. Create a multi_pack_index bit in the packed_git struct that is
one if and only if the pack was loaded from a multi-pack-index.
2. Skip packs with the multi_pack_index bit when doing object
lookups and abbreviations. These algorithms already check the
multi-pack-index before the packed_git struct. This has a very
small performance hit, as we need to walk more packed_git
structs. This is acceptable, since these operations run binary
search on the other packs, so this walk-and-ignore logic is
very fast by comparison.
3. When closing a multi-pack-index file, do not close its packs,
as those packs will be closed using close_all_packs(). In some
cases, such as 'git repack', we run 'close_midx()' without also
closing the packs, so we need to un-set the multi_pack_index bit
in those packs. This is necessary, and caught by running
t6501-freshen-objects.sh with GIT_TEST_MULTI_PACK_INDEX=1.
To manually test this change, I inserted trace2 logging into
close_pack_fd() and set pack_max_fds to 10, then ran 'git rev-list
--all --objects' on a copy of the Git repo with 300+ pack-files and
a multi-pack-index. The logs verified the packs are closed as
we read them beyond the file descriptor limit.
Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx>
---
midx.c | 20 ++++++++++++++------
object-store.h | 9 ++-------
packfile.c | 28 ++++++++--------------------
sha1-name.c | 6 ++++++
4 files changed, 30 insertions(+), 33 deletions(-)
diff --git a/midx.c b/midx.c
index 8b8faec35a..e7e1fe4d65 100644
--- a/midx.c
+++ b/midx.c
@@ -192,10 +192,8 @@ void close_midx(struct multi_pack_index *m)
m->fd = -1;
for (i = 0; i < m->num_packs; i++) {
- if (m->packs[i]) {
- close_pack(m->packs[i]);
- free(m->packs[i]);
- }
+ if (m->packs[i])
+ m->packs[i]->multi_pack_index = 0;
}
FREE_AND_NULL(m->packs);
FREE_AND_NULL(m->pack_names);
@@ -204,6 +202,7 @@ void close_midx(struct multi_pack_index *m)
int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t pack_int_id)
{
struct strbuf pack_name = STRBUF_INIT;
+ struct packed_git *p;
if (pack_int_id >= m->num_packs)
die(_("bad pack-int-id: %u (%u total packs)"),
@@ -215,9 +214,18 @@ int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t
strbuf_addf(&pack_name, "%s/pack/%s", m->object_dir,
m->pack_names[pack_int_id]);
- m->packs[pack_int_id] = add_packed_git(pack_name.buf, pack_name.len, m->local);
+ p = add_packed_git(pack_name.buf, pack_name.len, m->local);
strbuf_release(&pack_name);
- return !m->packs[pack_int_id];
+
+ if (!p)
+ return 1;
+
+ p->multi_pack_index = 1;
+ m->packs[pack_int_id] = p;
+ install_packed_git(r, p);
+ list_add_tail(&p->mru, &r->objects->packed_git_mru);
+
+ return 0;
}
int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m, uint32_t *result)
diff --git a/object-store.h b/object-store.h
index b086f5ecdb..7acbc7fffe 100644
--- a/object-store.h
+++ b/object-store.h
@@ -76,7 +76,8 @@ struct packed_git {
pack_keep_in_core:1,
freshened:1,
do_not_close:1,
- pack_promisor:1;
+ pack_promisor:1,
+ multi_pack_index:1;
unsigned char hash[GIT_MAX_RAWSZ];
struct revindex_entry *revindex;
/* something like ".git/objects/pack/xxxxx.pack" */
@@ -128,12 +129,6 @@ struct raw_object_store {
/* A most-recently-used ordered version of the packed_git list. */
struct list_head packed_git_mru;
- /*
- * A linked list containing all packfiles, starting with those
- * contained in the multi_pack_index.
- */
- struct packed_git *all_packs;
-
/*
* A fast, rough count of the number of objects in the repository.
* These two fields are not meant for direct access. Use
diff --git a/packfile.c b/packfile.c
index 7b94a14726..060de420d1 100644
--- a/packfile.c
+++ b/packfile.c
@@ -994,8 +994,6 @@ static void prepare_packed_git(struct repository *r)
}
rearrange_packed_git(r);
- r->objects->all_packs = NULL;
-
prepare_packed_git_mru(r);
r->objects->packed_git_initialized = 1;
}
@@ -1026,26 +1024,16 @@ struct multi_pack_index *get_multi_pack_index(struct repository *r)
struct packed_git *get_all_packs(struct repository *r)
{
- prepare_packed_git(r);
-
- if (!r->objects->all_packs) {
- struct packed_git *p = r->objects->packed_git;
- struct multi_pack_index *m;
-
- for (m = r->objects->multi_pack_index; m; m = m->next) {
- uint32_t i;
- for (i = 0; i < m->num_packs; i++) {
- if (!prepare_midx_pack(r, m, i)) {
- m->packs[i]->next = p;
- p = m->packs[i];
- }
- }
- }
+ struct multi_pack_index *m;
- r->objects->all_packs = p;
+ prepare_packed_git(r);
+ for (m = r->objects->multi_pack_index; m; m = m->next) {
+ uint32_t i;
+ for (i = 0; i < m->num_packs; i++)
+ prepare_midx_pack(r, m, i);
}
- return r->objects->all_packs;
+ return r->objects->packed_git;
}
struct list_head *get_packed_git_mru(struct repository *r)
@@ -2004,7 +1992,7 @@ int find_pack_entry(struct repository *r, const struct object_id *oid, struct pa
list_for_each(pos, &r->objects->packed_git_mru) {
struct packed_git *p = list_entry(pos, struct packed_git, mru);
- if (fill_pack_entry(oid, e, p)) {
+ if (!p->multi_pack_index && fill_pack_entry(oid, e, p)) {
list_move(&p->mru, &r->objects->packed_git_mru);
return 1;
}
diff --git a/sha1-name.c b/sha1-name.c
index 07c71a7567..42ac1c5bb6 100644
--- a/sha1-name.c
+++ b/sha1-name.c
@@ -157,6 +157,9 @@ static void unique_in_pack(struct packed_git *p,
uint32_t num, i, first = 0;
const struct object_id *current = NULL;
+ if (p->multi_pack_index)
+ return;
+
if (open_pack_index(p) || !p->num_objects)
return;
@@ -589,6 +592,9 @@ static void find_abbrev_len_for_pack(struct packed_git *p,
struct object_id oid;
const struct object_id *mad_oid;
+ if (p->multi_pack_index)
+ return;
+
if (open_pack_index(p) || !p->num_objects)
return;
--
gitgitgadget
