OpenZFS 8156 - dbuf_evict_notify() does not need dbuf_evict_lock

Authored by: Matthew Ahrens <[email protected]>
Reviewed by: Dan Kimmel <[email protected]>
Reviewed by: Paul Dagnelie <[email protected]>
Approved by: Robert Mustacchi <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Ported-by: Giuseppe Di Natale <[email protected]>

dbuf_evict_notify() holds the dbuf_evict_lock while checking if it should
do the eviction itself (because the evict thread is not able to keep up).
This can result in massive lock contention.  It isn't necessary to hold
the lock, because if we make the wrong choice occasionally, nothing bad
will happen. This commit results in a ~60% performance improvement for
ARC-cached sequential reads.

OpenZFS-issue: https://www.illumos.org/issues/8156
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/f73e5d9
Closes #6204

1 files changed, 7 insertions, 11 deletions

diff --git a/module/zfs/dbuf.c b/module/zfs/dbuf.c
index 611382686..70cc3108d 100644
--- a/module/zfs/dbuf.c
+++ b/module/zfs/dbuf.c
@@ -596,19 +596,15 @@ dbuf_evict_notify(void)
 	if (tsd_get(zfs_dbuf_evict_key) != NULL)
 		return;
 
+	/*
+	 * We check if we should evict without holding the dbuf_evict_lock,
+	 * because it's OK to occasionally make the wrong decision here,
+	 * and grabbing the lock results in massive lock contention.
+	 */
 	if (refcount_count(&dbuf_cache_size) > dbuf_cache_max_bytes) {
-		boolean_t evict_now = B_FALSE;
-
-		mutex_enter(&dbuf_evict_lock);
-		if (refcount_count(&dbuf_cache_size) > dbuf_cache_max_bytes) {
-			evict_now = dbuf_cache_above_hiwater();
-			cv_signal(&dbuf_evict_cv);
-		}
-		mutex_exit(&dbuf_evict_lock);
-
-		if (evict_now) {
+		if (dbuf_cache_above_hiwater())
 			dbuf_evict_one();
-		}
+		cv_signal(&dbuf_evict_cv);
 	}
 }