Minimize aggsum_compare(&arc_size, arc_c) calls.

For busy ARC situation when arc_size close to arc_c is desired.  But
then it is quite likely that aggsum_compare(&arc_size, arc_c) will need
to flush per-CPU buckets to find exact comparison result.  Doing that
often in a hot path penalizes whole idea of aggsum usage there, since it
replaces few simple atomic additions with dozens of lock acquisitions.

Replacing aggsum_compare() with aggsum_upper_bound() in code increasing
arc_p when ARC is growing (arc_size < arc_c) according to PMC profiles
allows to save ~5% of CPU time in aggsum code during sequential write
to 12 ZVOLs with 16KB block size on large dual-socket system.

I suppose there some minor arc_p behavior change due to lower precision
of the new code, but I don't think it is a big deal, since it should
affect only very small window in time (aggsum buckets are flushed every
second) and in ARC size (buckets are limited to 10 average ARC blocks
per CPU).

Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Allan Jude <allanjude@freebsd.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:  Alexander Motin <mav@FreeBSD.org>
Closes #8901

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index a7e7d26996f8c7baa6d728921d1039a3ba5f2dff..720365c4a9352396b69d39e4dd560c4653dfea7e 100644 (file)
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -5606,7 +5606,7 @@ arc_get_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag)
                 * If we are growing the cache, and we are adding anonymous
                 * data, and we have outgrown arc_p, update arc_p
                 */
-               if (aggsum_compare(&arc_size, arc_c) < 0 &&
+               if (aggsum_upper_bound(&arc_size) < arc_c &&
                    hdr->b_l1hdr.b_state == arc_anon &&
                    (zfs_refcount_count(&arc_anon->arcs_size) +
                    zfs_refcount_count(&arc_mru->arcs_size) > arc_p))