Allow scaling of arc in proportion to pagecache

When multiple filesystems are in use, memory pressure causes arc_cache
to collapse to a minimum. Allow arc_cache to maintain proportional size
even when hit rates are disproportionate. We do this only via evictable
size from the kernel shrinker, thus it's only in effect under memory
pressure.

AKAMAI: zfs: CR 3695072

Signed-off-by: Debabrata Banerjee <dbanerje@akamai.com>

man/man5/zfs-module-parameters.5

@@ -699,6 +699,23 @@ log2(fraction of arc to reclaim)
 Default value: \fB5\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_arc_pc_frac\fR (int)
+.ad
+.RS 12n
+n/128 fraction of pagecache to reclaim arc to
+
+This value allows the arc cache to compete with the pagecache even though
+there may be varying pressure on each. For example, scanning files through
+pagecache while arc cache is idle would cause the arc cache to collapse to
+minimum size. This value is specified as a fraction of pagecache size where
+that fraction may exceed 1. This only operates during memory reclaim.
+.sp
+Default value: \fB0\fR (disabled).
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/arc.c

@@ -319,6 +319,11 @@ static int		arc_p_min_shift = 4;
 /* log2(fraction of arc to reclaim) */
 static int		arc_shrink_shift = 7;
 
+/* n/128 fraction of pagecache to reclaim arc to */
+#ifdef _KERNEL
+static uint		zfs_arc_pc_frac = 0;
+#endif
+
 /*
  * log2(fraction of ARC which must be free to allow growing).
  * I.e. If there is less than arc_c >> arc_no_grow_shift free memory,
@@ -4350,10 +4355,19 @@ arc_evictable_memory(void)
 	    refcount_count(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]);
 	uint64_t arc_dirty = MAX((int64_t)arc_size - (int64_t)arc_clean, 0);
 
-	if (arc_dirty >= arc_c_min)
+	/*
+	 * Scale reported evictable memory in proportion to page cache, cap
+	 * at specified min/max.
+	 */
+
+	uint64_t min = (ptob(global_page_state(NR_FILE_PAGES)) >> 7) *
+	    zfs_arc_pc_frac;
+	min = MAX(arc_c_min, MIN(arc_c_max, min));
+
+	if (arc_dirty >= min)
 		return (arc_clean);
 
-	return (MAX((int64_t)arc_size - (int64_t)arc_c_min, 0));
+	return (MAX((int64_t)arc_size - (int64_t)min, 0));
 }
 
 /*
@@ -7725,6 +7739,10 @@ 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)");
 
+module_param(zfs_arc_pc_frac, uint, 0644);
+MODULE_PARM_DESC(zfs_arc_pc_frac,
+	"n/128 fraction of pagecache to reclaim arc to");
+
 module_param(zfs_arc_p_min_shift, int, 0644);
 MODULE_PARM_DESC(zfs_arc_p_min_shift, "arc_c shift to calc min/max arc_p");