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

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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 <mahrens@delphix.com>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/7004
OpenZFS-commit: https://github.com/openzfs/openzfs/pull/109
Closes #4641
Closes #4972
This commit is contained in:
Matthew Ahrens
2016-07-20 15:42:13 -07:00
committed by Brian Behlendorf
parent 8bea981504
commit 2bce8049c3
8 changed files with 146 additions and 19 deletions
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module/zfs/dmu_tx.c
+3 -4
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@@ -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.
*/
#include <sys/dmu.h>
@@ -794,15 +794,14 @@ dmu_tx_hold_zap(dmu_tx_t *tx, uint64_t object, int add, const char *name)
* access the name in this fat-zap so that we'll check
* for i/o errors to the leaf blocks, etc.
*/
err = zap_lookup(dn->dn_objset, dn->dn_object, name,
8, 0, NULL);
err = zap_lookup_by_dnode(dn, name, 8, 0, NULL);
if (err == EIO) {
tx->tx_err = err;
return;
}
}
err = zap_count_write(dn->dn_objset, dn->dn_object, name, add,
err = zap_count_write_by_dnode(dn, name, add,
&txh->txh_space_towrite, &txh->txh_space_tooverwrite);
/*