OpenZFS 8156 - dbuf_evict_notify() does not need dbuf_evict_lock

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
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

diff --git a/module/zfs/dbuf.c b/module/zfs/dbuf.c
index 611382686c89641c397a2628e0ed6eb9e04783a4..70cc3108d0ace6fae72248b5cd75dda8cd15dbae 100644 (file)
--- 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);
        }
 }