diff --git a/src/runtime/hashmap.go b/src/runtime/hashmap.go
index 086d374a1e6..1f2dafa91ee 100644
--- a/src/runtime/hashmap.go
+++ b/src/runtime/hashmap.go
@@ -958,6 +958,11 @@ func growWork(t *maptype, h *hmap, bucket uintptr) {
 	}
 }
 
+func bucketEvacuated(t *maptype, h *hmap, bucket uintptr) bool {
+	b := (*bmap)(add(h.oldbuckets, bucket*uintptr(t.bucketsize)))
+	return evacuated(b)
+}
+
 func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 	b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
 	newbit := h.noldbuckets()
@@ -1098,7 +1103,16 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 	// Advance evacuation mark
 	if oldbucket == h.nevacuate {
 		h.nevacuate = oldbucket + 1
-		if oldbucket+1 == newbit { // newbit == # of oldbuckets
+		// Experiments suggest that 1024 is overkill by at least an order of magnitude.
+		// Put it in there as a safeguard anyway, to ensure O(1) behavior.
+		stop := h.nevacuate + 1024
+		if stop > newbit {
+			stop = newbit
+		}
+		for h.nevacuate != stop && bucketEvacuated(t, h, h.nevacuate) {
+			h.nevacuate++
+		}
+		if h.nevacuate == newbit { // newbit == # of oldbuckets
 			// Growing is all done. Free old main bucket array.
 			h.oldbuckets = nil
 			// Can discard old overflow buckets as well.