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:
Benchmark 1: git fetch --prune --no-write-fetch-head +refs/*:refs/* (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: git fetch --prune --no-write-fetch-head +refs/*:refs/* (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
'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)' ran
1.13 ± 0.04 times faster than 'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)'
Signed-off-by: Patrick Steinhardt <ps@xxxxxx>
---
refs/files-backend.c | 34 ++++++++++++++++++++++++++++------
1 file changed, 28 insertions(+), 6 deletions(-)
diff --git a/refs/files-backend.c b/refs/files-backend.c
index f3428a9f12..0457ecdb42 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -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,
--
2.35.1
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