OpenZFS 7004 - dmu_tx_hold_zap() does dnode_hold() 7x on same object

Using a benchmark which has 32 threads creating 2 million files in the
same directory, on a machine with 16 CPU cores, I observed poor
performance. I noticed that dmu_tx_hold_zap() was using about 30% of
all CPU, and doing dnode_hold() 7 times on the same object (the ZAP
object that is being held).

dmu_tx_hold_zap() keeps a hold on the dnode_t the entire time it is
running, in dmu_tx_hold_t:txh_dnode, so it would be nice to use the
dnode_t that we already have in hand, rather than repeatedly calling
dnode_hold(). To do this, we need to pass the dnode_t down through
all the intermediate calls that dmu_tx_hold_zap() makes, making these
routines take the dnode_t* rather than an objset_t* and a uint64_t
object number. In particular, the following routines will need to have
analogous *_by_dnode() variants created:

dmu_buf_hold_noread()
dmu_buf_hold()
zap_lookup()
zap_lookup_norm()
zap_count_write()
zap_lockdir()
zap_count_write()

This can improve performance on the benchmark described above by 100%,
from 30,000 file creations per second to 60,000. (This improvement is on
top of that provided by working around the object allocation issue. Peak
performance of ~90,000 creations per second was observed with 8 CPUs;
adding CPUs past that decreased performance due to lock contention.) The
CPU used by dmu_tx_hold_zap() was reduced by 88%, from 340 CPU-seconds
to 40 CPU-seconds.

Sponsored by: Intel Corp.

Signed-off-by: Matthew Ahrens <[email protected]>
Signed-off-by: Ned Bass <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>

OpenZFS-issue: https://www.illumos.org/issues/7004
OpenZFS-commit: https://github.com/openzfs/openzfs/pull/109
Closes #4641
Closes #4972

1 files changed, 16 insertions, 1 deletions

diff --git a/module/zfs/dbuf.c b/module/zfs/dbuf.c
index d89d030d3..d297506e9 100644
--- a/module/zfs/dbuf.c
+++ b/module/zfs/dbuf.c
@@ -21,7 +21,7 @@
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
- * Copyright (c) 2012, 2015 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2016 by Delphix. All rights reserved.
  * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
  * Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
  */
@@ -2843,6 +2843,21 @@ dmu_buf_get_objset(dmu_buf_t *db)
 	return (dbi->db_objset);
 }
 
+dnode_t *
+dmu_buf_dnode_enter(dmu_buf_t *db)
+{
+	dmu_buf_impl_t *dbi = (dmu_buf_impl_t *)db;
+	DB_DNODE_ENTER(dbi);
+	return (DB_DNODE(dbi));
+}
+
+void
+dmu_buf_dnode_exit(dmu_buf_t *db)
+{
+	dmu_buf_impl_t *dbi = (dmu_buf_impl_t *)db;
+	DB_DNODE_EXIT(dbi);
+}
+
 static void
 dbuf_check_blkptr(dnode_t *dn, dmu_buf_impl_t *db)
 {