prepare_packed_git(): sort packs by age and localness.

When accessing objects, we first look for them in packs that
are linked together in the reverse order of discovery.

Since younger packs tend to contain more recent objects, which
are more likely to be accessed often, and local packs tend to
contain objects more relevant to our specific projects, sort the
list of packs before starting to access them.  In addition,
favoring local packs over the ones borrowed from alternates can
be a win when alternates are mounted on network file systems.

Signed-off-by: Junio C Hamano <junkio@cox.net>

cache.h

@@ -372,6 +372,7 @@ extern struct packed_git {
 	struct packed_git *next;
 	struct pack_window *windows;
 	uint32_t *index_base;
+	time_t mtime;
 	off_t index_size;
 	off_t pack_size;
 	int pack_fd;

sha1_file.c

@@ -739,6 +739,7 @@ struct packed_git *add_packed_git(char *path, int path_len, int local)
 	p->windows = NULL;
 	p->pack_fd = -1;
 	p->pack_local = local;
+	p->mtime = st.st_mtime;
 	if ((path_len > 44) && !get_sha1_hex(path + path_len - 44, sha1))
 		hashcpy(p->sha1, sha1);
 	return p;
@@ -823,6 +824,60 @@ static void prepare_packed_git_one(char *objdir, int local)
 	closedir(dir);
 }
 
+static int sort_pack(const void *a_, const void *b_)
+{
+	struct packed_git *a = *((struct packed_git **)a_);
+	struct packed_git *b = *((struct packed_git **)b_);
+	int st;
+
+	/*
+	 * Local packs tend to contain objects specific to our
+	 * variant of the project than remote ones.  In addition,
+	 * remote ones could be on a network mounted filesystem.
+	 * Favor local ones for these reasons.
+	 */
+	st = a->pack_local - b->pack_local;
+	if (st)
+		return -st;
+
+	/*
+	 * Younger packs tend to contain more recent objects,
+	 * and more recent objects tend to get accessed more
+	 * often.
+	 */
+	if (a->mtime < b->mtime)
+		return 1;
+	else if (a->mtime == b->mtime)
+		return 0;
+	return -1;
+}
+
+static void rearrange_packed_git(void)
+{
+	struct packed_git **ary, *p;
+	int i, n;
+
+	for (n = 0, p = packed_git; p; p = p->next)
+		n++;
+	if (n < 2)
+		return;
+
+	/* prepare an array of packed_git for easier sorting */
+	ary = xcalloc(n, sizeof(struct packed_git *));
+	for (n = 0, p = packed_git; p; p = p->next)
+		ary[n++] = p;
+
+	qsort(ary, n, sizeof(struct packed_git *), sort_pack);
+
+	/* link them back again */
+	for (i = 0; i < n - 1; i++)
+		ary[i]->next = ary[i + 1];
+	ary[n - 1]->next = NULL;
+	packed_git = ary[0];
+
+	free(ary);
+}
+
 static int prepare_packed_git_run_once = 0;
 void prepare_packed_git(void)
 {
@@ -837,6 +892,7 @@ void prepare_packed_git(void)
 		prepare_packed_git_one(alt->base, 0);
 		alt->name[-1] = '/';
 	}
+	rearrange_packed_git();
 	prepare_packed_git_run_once = 1;
 }