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OpenZFS 9188 - increase size of dbuf cache to reduce indirect block decompression 

With compressed ARC (bug 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%).

Porting Notes:
* Added modules options to zfs-module-parameters.5 man page.
* Preserved scaling based on target ARC size rather than max ARC size.

Authored by: George Wilson <george.wilson@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/9188
OpenZFS-commit: https://github.com/openzfs/openzfs/pull/564
Upstream bug: DLPX-46942
Closes #7273
This commit is contained in:
Brian Behlendorf 2018-03-13 10:52:48 -07:00 committed by GitHub
parent a6cc97566c
commit de4f8d5d26
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2 changed files with 66 additions and 11 deletions
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51
man/man5/zfs-module-parameters.5
View File

@ -25,6 +25,57 @@ Description of the different parameters to the ZFS module.
.sp
.LP
.sp
.ne 2
.na
\fBdbuf_cache_max_bytes\fR (ulong)
.ad
.RS 12n
Maximum size in bytes of the dbuf cache. When \fB0\fR this value will default
to \fB1/2^dbuf_cache_shift\fR (1/32) of the target ARC size, otherwise the
provided value in bytes will be used. The behavior of the dbuf cache and its
associated settings can be observed via the \fB/proc/spl/kstat/zfs/dbufstats\fR
kstat.
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
\fBdbuf_cache_hiwater_pct\fR (uint)
.ad
.RS 12n
The percentage over \fBdbuf_cache_max_bytes\fR when dbufs must be evicted
directly.
.sp
Default value: \fB10\fR%.
.RE
.sp
.ne 2
.na
\fBdbuf_cache_lowater_pct\fR (uint)
.ad
.RS 12n
The percentage below \fBdbuf_cache_max_bytes\fR when the evict thread stops
evicting dbufs.
.sp
Default value: \fB10\fR%.
.RE
.sp
.ne 2
.na
\fBdbuf_cache_shift\fR (int)
.ad
.RS 12n
Set the size of the dbuf cache, \fBdbuf_cache_max_bytes\fR, to a log2 fraction
of the target arc size.
.sp
Default value: \fB5\fR.
.RE
.sp
.ne 2
.na

26
module/zfs/dbuf.c
View File

@ -186,10 +186,10 @@ static boolean_t dbuf_evict_thread_exit;
*/
static multilist_t *dbuf_cache;
static refcount_t dbuf_cache_size;
unsigned long dbuf_cache_max_bytes = 100 * 1024 * 1024;
unsigned long 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:
@ -561,7 +561,7 @@ static inline unsigned long
dbuf_cache_target_bytes(void)
{
return MIN(dbuf_cache_max_bytes,
arc_target_bytes() >> dbuf_cache_max_shift);
arc_target_bytes() >> dbuf_cache_shift);
}
static inline uint64_t
@ -787,11 +787,15 @@ retry:
dbuf_stats_init(h);
/*
* 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 target size of the ARC. If
* the value has been specified as a module option and it's not
* greater than the target size of the ARC, then we honor that value.
*/
dbuf_cache_max_bytes = MIN(dbuf_cache_max_bytes,
arc_target_bytes() >> dbuf_cache_max_shift);
if (dbuf_cache_max_bytes == 0 ||
dbuf_cache_max_bytes >= arc_target_bytes()) {
dbuf_cache_max_bytes = arc_target_bytes() >> dbuf_cache_shift;
}
/*
* All entries are queued via taskq_dispatch_ent(), so min/maxalloc
@ -4216,8 +4220,8 @@ MODULE_PARM_DESC(dbuf_cache_lowater_pct,
"Percentage below dbuf_cache_max_bytes when the evict thread stops "
"evicting dbufs.");
module_param(dbuf_cache_max_shift, int, 0644);
MODULE_PARM_DESC(dbuf_cache_max_shift,
"Cap the size of the dbuf cache to a log2 fraction of arc size.");
module_param(dbuf_cache_shift, int, 0644);
MODULE_PARM_DESC(dbuf_cache_shift,
"Set the size of the dbuf cache to a log2 fraction of arc size.");
/* END CSTYLED */
#endif