Disable arc_p adapt dampener by default

It's unclear why adjustments to arc_p need to be dampened as they are in
arc_adjust. With that said, it's removal significantly improves the arc's
ability to "warm up" to a given workload. Thus, I'm disabling by default
until its usefulness is better understood.

Signed-off-by: Prakash Surya <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Issue #2110

2 files changed, 23 insertions, 2 deletions

diff --git a/man/man5/zfs-module-parameters.5 b/man/man5/zfs-module-parameters.5
index 2acf68383..e0d44d22c 100644
--- a/man/man5/zfs-module-parameters.5
+++ b/man/man5/zfs-module-parameters.5
@@ -307,6 +307,17 @@ Use \fB1\fR for yes (default) and \fB0\fR to disable.
 .sp
 .ne 2
 .na
+\fBzfs_arc_p_dampener_disable\fR (int)
+.ad
+.RS 12n
+Disable arc_p adapt dampener
+.sp
+Use \fB1\fR for yes (default) and \fB0\fR to disable.
+.RE
+
+.sp
+.ne 2
+.na
 \fBzfs_arc_shrink_shift\fR (int)
 .ad
 .RS 12n
diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index f66eaa407..68496783d 100644
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -175,6 +175,9 @@ int zfs_arc_grow_retry = 5;
 /* disable anon data aggressively growing arc_p */
 int zfs_arc_p_aggressive_disable = 1;
 
+/* disable arc_p adapt dampener in arc_adapt */
+int zfs_arc_p_dampener_disable = 1;
+
 /* log2(fraction of arc to reclaim) */
 int zfs_arc_shrink_shift = 5;
 
@@ -2633,7 +2636,9 @@ arc_adapt(int bytes, arc_state_t *state)
 	if (state == arc_mru_ghost) {
 		mult = ((arc_mru_ghost->arcs_size >= arc_mfu_ghost->arcs_size) ?
 		    1 : (arc_mfu_ghost->arcs_size/arc_mru_ghost->arcs_size));
-		mult = MIN(mult, 10); /* avoid wild arc_p adjustment */
+
+		if (!zfs_arc_p_dampener_disable)
+			mult = MIN(mult, 10); /* avoid wild arc_p adjustment */
 
 		arc_p = MIN(arc_c, arc_p + bytes * mult);
 	} else if (state == arc_mfu_ghost) {
@@ -2641,7 +2646,9 @@ arc_adapt(int bytes, arc_state_t *state)
 
 		mult = ((arc_mfu_ghost->arcs_size >= arc_mru_ghost->arcs_size) ?
 		    1 : (arc_mru_ghost->arcs_size/arc_mfu_ghost->arcs_size));
-		mult = MIN(mult, 10);
+
+		if (!zfs_arc_p_dampener_disable)
+			mult = MIN(mult, 10);
 
 		delta = MIN(bytes * mult, arc_p);
 		arc_p = MAX(0, arc_p - delta);
@@ -5554,6 +5561,9 @@ MODULE_PARM_DESC(zfs_arc_grow_retry, "Seconds before growing arc size");
 module_param(zfs_arc_p_aggressive_disable, int, 0644);
 MODULE_PARM_DESC(zfs_arc_p_aggressive_disable, "disable aggressive arc_p grow");
 
+module_param(zfs_arc_p_dampener_disable, int, 0644);
+MODULE_PARM_DESC(zfs_arc_p_dampener_disable, "disable arc_p adapt dampener");
+
 module_param(zfs_arc_shrink_shift, int, 0644);
 MODULE_PARM_DESC(zfs_arc_shrink_shift, "log2(fraction of arc to reclaim)");