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refs/files-backend: optimize reading of symbolic refs 

When reading references via `files_read_raw_ref()` we always consult
both the loose reference, and if that wasn't found, we also consult the
packed-refs file. While this makes sense to read a generic reference, it
is wasteful in the case where we only care about symbolic references:
the packed-refs backend does not support them, and thus it cannot ever
return one for us.

Special-case reading of symbolic references for the files backend such
that we always skip asking the packed-refs backend.

We use `refs_read_symbolic_ref()` extensively to determine whether we
need to skip updating local symbolic references during a fetch, which is
why the change results in a significant speedup when doing fetches in
repositories with huge numbers of references. The following benchmark
executes a mirror-fetch in a repository with about 2 million references
via `git fetch --prune --no-write-fetch-head +refs/*:refs/*`:

    Benchmark 1: HEAD~
      Time (mean ± σ):     68.372 s ±  2.344 s    [User: 65.629 s, System: 8.786 s]
      Range (min … max):   65.745 s … 70.246 s    3 runs

    Benchmark 2: HEAD
      Time (mean ± σ):     60.259 s ±  0.343 s    [User: 61.019 s, System: 7.245 s]
      Range (min … max):   60.003 s … 60.649 s    3 runs

    Summary
      'HEAD' ran
        1.13 ± 0.04 times faster than 'HEAD~'

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Patrick Steinhardt 2022-03-01 10:33:54 +01:00 committed by Junio C Hamano
parent 1553f5e76c
commit 0a7b38707d
1 changed files with 28 additions and 6 deletions
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34
refs/files-backend.c
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@ -338,9 +338,9 @@ static struct ref_cache *get_loose_ref_cache(struct files_ref_store *refs)
return refs->loose;
}
static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
struct object_id *oid, struct strbuf *referent,
unsigned int *type, int *failure_errno)
static int read_ref_internal(struct ref_store *ref_store, const char *refname,
struct object_id *oid, struct strbuf *referent,
unsigned int *type, int *failure_errno, int skip_packed_refs)
{
struct files_ref_store *refs =
files_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
@ -381,7 +381,7 @@ static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
if (lstat(path, &st) < 0) {
int ignore_errno;
myerr = errno;
if (myerr != ENOENT)
if (myerr != ENOENT || skip_packed_refs)
goto out;
if (refs_read_raw_ref(refs->packed_ref_store, refname, oid,
referent, type, &ignore_errno)) {
@ -425,7 +425,8 @@ static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
* ref is supposed to be, there could still be a
* packed ref:
*/
if (refs_read_raw_ref(refs->packed_ref_store, refname, oid,
if (skip_packed_refs ||
refs_read_raw_ref(refs->packed_ref_store, refname, oid,
referent, type, &ignore_errno)) {
myerr = EISDIR;
goto out;
@ -470,6 +471,27 @@ static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
return ret;
}
static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
struct object_id *oid, struct strbuf *referent,
unsigned int *type, int *failure_errno)
{
return read_ref_internal(ref_store, refname, oid, referent, type, failure_errno, 0);
}
static int files_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
struct strbuf *referent)
{
struct object_id oid;
int failure_errno, ret;
unsigned int type;
ret = read_ref_internal(ref_store, refname, &oid, referent, &type, &failure_errno, 1);
if (ret)
return ret;
return !(type & REF_ISSYMREF);
}
int parse_loose_ref_contents(const char *buf, struct object_id *oid,
struct strbuf *referent, unsigned int *type,
int *failure_errno)
@ -3286,7 +3308,7 @@ struct ref_storage_be refs_be_files = {
files_ref_iterator_begin,
files_read_raw_ref,
NULL,
files_read_symbolic_ref,
files_reflog_iterator_begin,
files_for_each_reflog_ent,