Author: mav
Date: Mon Apr 16 04:15:25 2018
New Revision: 332552

  MFC r331711: MFV 331710:
  9188 increase size of dbuf cache to reduce indirect block decompression
  With compressed ARC (6950) we use up to 25% of our CPU to decompress indirect
  blocks, under a workload of random cached reads. To reduce this decompression
  cost, we would like to increase the size of the dbuf cache so that more
  indirect blocks can be stored uncompressed.
  If we are caching entire large files of recordsize=8K, the indirect blocks
  use 1/64th as much memory as the data blocks (assuming they have the same
  compression ratio). We suggest making the dbuf cache be 1/32nd of all memory,
  so that in this scenario we should be able to keep all the indirect blocks
  decompressed in the dbuf cache. (We want it to be more than the 1/64th that
  the indirect blocks would use because we need to cache other stuff in the
  dbuf cache as well.)
  In real world workloads, this won't help as dramatically as the example
  above, but we think it's still worth it because the risk of decreasing
  performance is low. The potential negative performance impact is that we
  will be slightly reducing the size of the ARC (by ~3%).
  Reviewed by: Dan Kimmel <>
  Reviewed by: Prashanth Sreenivasa <>
  Reviewed by: Paul Dagnelie <>
  Reviewed by: Sanjay Nadkarni <>
  Reviewed by: Allan Jude <>
  Reviewed by: Igor Kozhukhov <>
  Approved by: Garrett D'Amore <>
  Author: George Wilson <>

Directory Properties:
  stable/11/   (props changed)

Modified: stable/11/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dbuf.c
--- stable/11/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dbuf.c     Mon Apr 
16 04:14:42 2018        (r332551)
+++ stable/11/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dbuf.c     Mon Apr 
16 04:15:25 2018        (r332552)
@@ -85,10 +85,10 @@ static boolean_t dbuf_evict_thread_exit;
 static multilist_t *dbuf_cache;
 static refcount_t dbuf_cache_size;
-uint64_t dbuf_cache_max_bytes = 100 * 1024 * 1024;
+uint64_t dbuf_cache_max_bytes = 0;
-/* Cap the size of the dbuf cache to log2 fraction of arc size. */
-int dbuf_cache_max_shift = 5;
+/* Set the default size of the dbuf cache to log2 fraction of arc size. */
+int dbuf_cache_shift = 5;
  * The dbuf cache uses a three-stage eviction policy:
@@ -138,8 +138,8 @@ uint_t dbuf_cache_lowater_pct = 10;
 SYSCTL_QUAD(_vfs_zfs, OID_AUTO, dbuf_cache_max_bytes, CTLFLAG_RWTUN,
     &dbuf_cache_max_bytes, 0, "dbuf cache size in bytes");
-SYSCTL_INT(_vfs_zfs, OID_AUTO, dbuf_cache_max_shift, CTLFLAG_RDTUN,
-    &dbuf_cache_max_shift, 0, "dbuf size as log2 fraction of ARC");
+SYSCTL_INT(_vfs_zfs, OID_AUTO, dbuf_cache_shift, CTLFLAG_RDTUN,
+    &dbuf_cache_shift, 0, "dbuf cache size as log2 fraction of ARC");
 SYSCTL_UINT(_vfs_zfs, OID_AUTO, dbuf_cache_hiwater_pct, CTLFLAG_RWTUN,
     &dbuf_cache_hiwater_pct, 0, "max percents above the dbuf cache size");
 SYSCTL_UINT(_vfs_zfs, OID_AUTO, dbuf_cache_lowater_pct, CTLFLAG_RWTUN,
@@ -610,11 +610,15 @@ retry:
                mutex_init(&h->hash_mutexes[i], NULL, MUTEX_DEFAULT, NULL);
-        * Setup the parameters for the dbuf cache. We cap the size of the
-        * dbuf cache to 1/32nd (default) of the size of the ARC.
+        * Setup the parameters for the dbuf cache. We set the size of the
+        * dbuf cache to 1/32nd (default) of the size of the ARC. If the value
+        * has been set in /etc/system and it's not greater than the size of
+        * the ARC, then we honor that value.
-       dbuf_cache_max_bytes = MIN(dbuf_cache_max_bytes,
-           arc_max_bytes() >> dbuf_cache_max_shift);
+       if (dbuf_cache_max_bytes == 0 ||
+           dbuf_cache_max_bytes >= arc_max_bytes())  {
+               dbuf_cache_max_bytes = arc_max_bytes() >> dbuf_cache_shift;
+       }
         * All entries are queued via taskq_dispatch_ent(), so min/maxalloc
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