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, 8 insertions, 1 deletions

diff --git a/include/sys/zap.h b/include/sys/zap.h
index 700947397..c4169029a 100644
--- a/include/sys/zap.h
+++ b/include/sys/zap.h
@@ -236,7 +236,14 @@ int zap_prefetch(objset_t *os, uint64_t zapobj, const char *name);
 int zap_prefetch_uint64(objset_t *os, uint64_t zapobj, const uint64_t *key,
     int key_numints);
 
-int zap_count_write(objset_t *os, uint64_t zapobj, const char *name,
+int zap_lookup_by_dnode(dnode_t *dn, const char *name,
+    uint64_t integer_size, uint64_t num_integers, void *buf);
+int zap_lookup_norm_by_dnode(dnode_t *dn, const char *name,
+    uint64_t integer_size, uint64_t num_integers, void *buf,
+    matchtype_t mt, char *realname, int rn_len,
+    boolean_t *ncp);
+
+int zap_count_write_by_dnode(dnode_t *dn, const char *name,
     int add, uint64_t *towrite, uint64_t *tooverwrite);
 
 /*