Am 29.06.21 um 22:53 schrieb Eric Wong:
> With many alternates, the duplicate check in alt_odb_usable()
> wastes many cycles doing repeated fspathcmp() on every existing
> alternate. Use a khash to speed up lookups by odb->path.
>
> Since the kh_put_* API uses the supplied key without
> duplicating it, we also take advantage of it to replace both
> xstrdup() and strbuf_release() in link_alt_odb_entry() with
> strbuf_detach() to avoid the allocation and copy.
>
> In a test repository with 50K alternates and each of those 50K
> alternates having one alternate each (for a total of 100K total
> alternates); this speeds up lookup of a non-existent blob from
> over 16 minutes to roughly 2.7 seconds on my busy workstation.
Yay for hashmaps! :)
> Note: all underlying git object directories were small and
> unpacked with only loose objects and no packs. Having to load
> packs increases times significantly.
>
> Signed-off-by: Eric Wong <e@xxxxxxxxx>
> ---
> object-file.c | 33 ++++++++++++++++++++++-----------
> object-store.h | 17 +++++++++++++++++
> object.c | 2 ++
> 3 files changed, 41 insertions(+), 11 deletions(-)
>
> diff --git a/object-file.c b/object-file.c
> index f233b440b2..304af3a172 100644
> --- a/object-file.c
> +++ b/object-file.c
> @@ -517,9 +517,9 @@ const char *loose_object_path(struct repository *r, struct strbuf *buf,
> */
> static int alt_odb_usable(struct raw_object_store *o,
> struct strbuf *path,
> - const char *normalized_objdir)
> + const char *normalized_objdir, khiter_t *pos)
> {
> - struct object_directory *odb;
> + int r;
>
> /* Detect cases where alternate disappeared */
> if (!is_directory(path->buf)) {
> @@ -533,14 +533,22 @@ static int alt_odb_usable(struct raw_object_store *o,
> * Prevent the common mistake of listing the same
> * thing twice, or object directory itself.
> */
> - for (odb = o->odb; odb; odb = odb->next) {
> - if (!fspathcmp(path->buf, odb->path))
> - return 0;
> + if (!o->odb_by_path) {
> + khiter_t p;
> +
> + o->odb_by_path = kh_init_odb_path_map();
> + assert(!o->odb->next);
> + p = kh_put_odb_path_map(o->odb_by_path, o->odb->path, &r);
So on the first run you not just create the hashmap, but you also
pre-populate it with the main object directory. Makes sense. The
hashmap wouldn't even be created in repositories without alternates.
> + if (r < 0) die_errno(_("kh_put_odb_path_map"));
Our other callers don't handle a negative return code because it would
indicate an allocation failure, and in our version we use ALLOC_ARRAY,
which dies on error. So you don't need that check here, but we better
clarify that in khash.h.
> + assert(r == 1); /* never used */
> + kh_value(o->odb_by_path, p) = o->odb;
> }
> if (!fspathcmp(path->buf, normalized_objdir))
> return 0;
> -
> - return 1;
> + *pos = kh_put_odb_path_map(o->odb_by_path, path->buf, &r);
> + if (r < 0) die_errno(_("kh_put_odb_path_map"));
Dito.
> + /* r: 0 = exists, 1 = never used, 2 = deleted */
> + return r == 0 ? 0 : 1;
The comment indicates that khash would be nicer to use if it had an
enum for the kh_put return values. Perhaps, but that should be done in
another series.
I like the solution in oidset.c to make this more readable, though: Call
the return value "added" instead of "r" and then a "return !added;"
makes sense without additional comments.
> }
>
> /*
> @@ -566,6 +574,7 @@ static int link_alt_odb_entry(struct repository *r, const char *entry,
> {
> struct object_directory *ent;
> struct strbuf pathbuf = STRBUF_INIT;
> + khiter_t pos;
>
> if (!is_absolute_path(entry) && relative_base) {
> strbuf_realpath(&pathbuf, relative_base, 1);
> @@ -587,23 +596,25 @@ static int link_alt_odb_entry(struct repository *r, const char *entry,
> while (pathbuf.len && pathbuf.buf[pathbuf.len - 1] == '/')
> strbuf_setlen(&pathbuf, pathbuf.len - 1);
>
> - if (!alt_odb_usable(r->objects, &pathbuf, normalized_objdir)) {
> + if (!alt_odb_usable(r->objects, &pathbuf, normalized_objdir, &pos)) {
> strbuf_release(&pathbuf);
> return -1;
> }
>
> CALLOC_ARRAY(ent, 1);
> - ent->path = xstrdup(pathbuf.buf);
> + /* pathbuf.buf is already in r->objects->odb_by_path */
Tricky stuff (to me), important comment.
> + ent->path = strbuf_detach(&pathbuf, NULL);
>
> /* add the alternate entry */
> *r->objects->odb_tail = ent;
> r->objects->odb_tail = &(ent->next);
> ent->next = NULL;
> + assert(r->objects->odb_by_path);
> + kh_value(r->objects->odb_by_path, pos) = ent;
>
> /* recursively add alternates */
> - read_info_alternates(r, pathbuf.buf, depth + 1);
> + read_info_alternates(r, ent->path, depth + 1);
>
> - strbuf_release(&pathbuf);
> return 0;
> }
>
> diff --git a/object-store.h b/object-store.h
> index ec32c23dcb..20c1cedb75 100644
> --- a/object-store.h
> +++ b/object-store.h
> @@ -7,6 +7,8 @@
> #include "oid-array.h"
> #include "strbuf.h"
> #include "thread-utils.h"
> +#include "khash.h"
> +#include "dir.h"
>
> struct object_directory {
> struct object_directory *next;
> @@ -30,6 +32,19 @@ struct object_directory {
> char *path;
> };
>
> +static inline int odb_path_eq(const char *a, const char *b)
> +{
> + return !fspathcmp(a, b);
> +}
This is not specific to the object store. It could be called fspatheq
and live in dir.h. Or dir.c -- a surprising amount of code seems to
necessary for that negation (https://godbolt.org/z/MY7Wda3a7). Anyway,
it's just an idea for another series.
> +
> +static inline int odb_path_hash(const char *str)
> +{
> + return ignore_case ? strihash(str) : __ac_X31_hash_string(str);
> +}
The internal Attractive Chaos (__ac_*) macros should be left confined
to khash.h, I think. Its alias kh_str_hash_func would be better
suited here.
Do we want to use the K&R hash function here at all, though? If we
use FNV-1 when ignoring case, why not also use it (i.e. strhash) when
respecting it? At least that's done in builtin/sparse-checkout.c,
dir.c and merge-recursive.c. This is just handwaving and yammering
about lack of symmetry, but I do wonder how your performance numbers
look with strhash. If it's fine then we could package this up as
fspathhash..
And I also wonder how it looks if you use strihash unconditionally.
I guess case collisions are usually rare and branching based on a
global variable may be more expensive than case folding..
Anyway, just ideas; kh_str_hash_func would be OK as well.
> +
> +KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
> + struct object_directory *, 1, odb_path_hash, odb_path_eq);
> +
> void prepare_alt_odb(struct repository *r);
> char *compute_alternate_path(const char *path, struct strbuf *err);
> typedef int alt_odb_fn(struct object_directory *, void *);
> @@ -116,6 +131,8 @@ struct raw_object_store {
> */
> struct object_directory *odb;
> struct object_directory **odb_tail;
> + kh_odb_path_map_t *odb_by_path;
> +
> int loaded_alternates;
>
> /*
> diff --git a/object.c b/object.c
> index 14188453c5..2b3c075a15 100644
> --- a/object.c
> +++ b/object.c
> @@ -511,6 +511,8 @@ static void free_object_directories(struct raw_object_store *o)
> free_object_directory(o->odb);
> o->odb = next;
> }
> + kh_destroy_odb_path_map(o->odb_by_path);
> + o->odb_by_path = NULL;
> }
>
> void raw_object_store_clear(struct raw_object_store *o)
>
