Reduce buf/dbuf mutex contention

Due to evidence of contention both the buf_hash_table and the
dbuf_hash_table sizes have been increased from 256 to 8192.

This increase in hash table size adds approximating 0.5M to
our fixed memory footprint.  This relatively small increase
is not expected to cause problems even on low memory machines.
This footprint will also become dynamic when the persistent
L2ARC support is finalized.  In the meanwhile, this small
change significantly reduces contention for certain workloads.

Signed-off-by: Chris Wedgwood <cw@f00f.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>
Closes #1291

include/sys/dbuf.h

@@ -233,7 +233,7 @@ typedef struct dmu_buf_impl {
 } dmu_buf_impl_t;
 
 /* Note: the dbuf hash table is exposed only for the mdb module */
-#define	DBUF_MUTEXES 256
+#define	DBUF_MUTEXES 8192
 #define	DBUF_HASH_MUTEX(h, idx) (&(h)->hash_mutexes[(idx) & (DBUF_MUTEXES-1)])
 typedef struct dbuf_hash_table {
 	uint64_t hash_table_mask;

module/zfs/arc.c

@@ -657,7 +657,7 @@ struct ht_lock {
 #endif
 };
 
-#define	BUF_LOCKS 256
+#define	BUF_LOCKS 8192
 typedef struct buf_hash_table {
 	uint64_t ht_mask;
 	arc_buf_hdr_t **ht_table;