MFC r267392:

Implement simple direct-mapped cache for popular filesystem identifiers to
avoid congestion on global mountlist_mtx mutex in vfs_busyfs(), while
traversing through the list of mount points.

This change significantly improves NFS server scalability, since it had
to do this translation for every request, and the global lock becomes quite
congested.

This code is more optimized for relatively small number of mount points.
On systems with hundreds of active mount points this simple cache may have
many collisions.  But the original traversal code in that case should also
behave much worse, so we are not loosing much.

1 files changed, 30 insertions, 0 deletions

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 8c3af46..52b4059 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -500,23 +500,53 @@ vfs_getvfs(fsid_t *fsid)
 /*
  * Lookup a mount point by filesystem identifier, busying it before
  * returning.
+ *
+ * To avoid congestion on mountlist_mtx, implement simple direct-mapped
+ * cache for popular filesystem identifiers.  The cache is lockess, using
+ * the fact that struct mount's are never freed.  In worst case we may
+ * get pointer to unmounted or even different filesystem, so we have to
+ * check what we got, and go slow way if so.
  */
 struct mount *
 vfs_busyfs(fsid_t *fsid)
 {
+#define	FSID_CACHE_SIZE	256
+	typedef struct mount * volatile vmp_t;
+	static vmp_t cache[FSID_CACHE_SIZE];
 	struct mount *mp;
 	int error;
+	uint32_t hash;
 
 	CTR2(KTR_VFS, "%s: fsid %p", __func__, fsid);
+	hash = fsid->val[0] ^ fsid->val[1];
+	hash = (hash >> 16 ^ hash) & (FSID_CACHE_SIZE - 1);
+	mp = cache[hash];
+	if (mp == NULL ||
+	    mp->mnt_stat.f_fsid.val[0] != fsid->val[0] ||
+	    mp->mnt_stat.f_fsid.val[1] != fsid->val[1])
+		goto slow;
+	if (vfs_busy(mp, 0) != 0) {
+		cache[hash] = NULL;
+		goto slow;
+	}
+	if (mp->mnt_stat.f_fsid.val[0] == fsid->val[0] &&
+	    mp->mnt_stat.f_fsid.val[1] == fsid->val[1])
+		return (mp);
+	else
+	    vfs_unbusy(mp);
+
+slow:
 	mtx_lock(&mountlist_mtx);
 	TAILQ_FOREACH(mp, &mountlist, mnt_list) {
 		if (mp->mnt_stat.f_fsid.val[0] == fsid->val[0] &&
 		    mp->mnt_stat.f_fsid.val[1] == fsid->val[1]) {
 			error = vfs_busy(mp, MBF_MNTLSTLOCK);
 			if (error) {
+				cache[hash] = NULL;
 				mtx_unlock(&mountlist_mtx);
 				return (NULL);
 			}
+			cache[hash] = mp;
 			return (mp);
 		}
 	}