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>

diff --git a/refs/files-backend.c b/refs/files-backend.c
index f3428a9f122147501dc2845ad16f7855a6620386..0457ecdb42dac658673519779f55f3d08ef50beb 100644 (file)
--- 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 @@ stat_ref:
        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 @@ stat_ref:
                 * 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 @@ out:
        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,