Soften pruning threshold on not evictable metadata

Previous code pruned 10% of dnodes once 3/4 of metadata appeared
unevictable.  On workloads with many millions of dnodes and little
other metadata it creates significant load spikes for many seconds
straight.  This change instead gradually increases pruning as
unevictable metadata grow above the 3/4, which may allow it to
stabilize at some level.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #16401 

1 files changed, 23 insertions, 8 deletions

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index 9bb7eeef3..78c2cf8ec 100644
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -4236,6 +4236,18 @@ arc_evict_adj(uint64_t frac, uint64_t total, uint64_t up, uint64_t down,
 }
 
 /*
+ * Calculate (x * multiplier / divisor) without unnecesary overflows.
+ */
+static uint64_t
+arc_mf(uint64_t x, uint64_t multiplier, uint64_t divisor)
+{
+	uint64_t q = (x / divisor);
+	uint64_t r = (x % divisor);
+
+	return ((q * multiplier) + ((r * multiplier) / divisor));
+}
+
+/*
  * Evict buffers from the cache, such that arcstat_size is capped by arc_c.
  */
 static uint64_t
@@ -4287,17 +4299,20 @@ arc_evict(void)
 	 */
 	int64_t prune = 0;
 	int64_t dn = wmsum_value(&arc_sums.arcstat_dnode_size);
+	int64_t nem = zfs_refcount_count(&arc_mru->arcs_size[ARC_BUFC_METADATA])
+	    + zfs_refcount_count(&arc_mfu->arcs_size[ARC_BUFC_METADATA])
+	    - zfs_refcount_count(&arc_mru->arcs_esize[ARC_BUFC_METADATA])
+	    - zfs_refcount_count(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]);
 	w = wt * (int64_t)(arc_meta >> 16) >> 16;
-	if (zfs_refcount_count(&arc_mru->arcs_size[ARC_BUFC_METADATA]) +
-	    zfs_refcount_count(&arc_mfu->arcs_size[ARC_BUFC_METADATA]) -
-	    zfs_refcount_count(&arc_mru->arcs_esize[ARC_BUFC_METADATA]) -
-	    zfs_refcount_count(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]) >
-	    w * 3 / 4) {
+	if (nem > w * 3 / 4) {
 		prune = dn / sizeof (dnode_t) *
 		    zfs_arc_dnode_reduce_percent / 100;
-	} else if (dn > arc_dnode_limit) {
-		prune = (dn - arc_dnode_limit) / sizeof (dnode_t) *
-		    zfs_arc_dnode_reduce_percent / 100;
+		if (nem < w && w > 4)
+			prune = arc_mf(prune, nem - w * 3 / 4, w / 4);
+	}
+	if (dn > arc_dnode_limit) {
+		prune = MAX(prune, (dn - arc_dnode_limit) / sizeof (dnode_t) *
+		    zfs_arc_dnode_reduce_percent / 100);
 	}
 	if (prune > 0)
 		arc_prune_async(prune);