a couple of comments

distributed/tests/test_rootish.py

@@ -84,6 +84,10 @@ def test_nearest_neighbor(abcde, num_Bs, BRootish):
     a1  a2  a3  a4  a5  a6  a7 a8  a9
      \  |  /  \ |  /  \ |  / \ |  /
         b1      b2      b3     b4
+
+    How these groups are classified depends on an implementation detail in the
+    scheduler since we're defining a relatively artificial cutoff requiring
+    root-ish groups to have at least 5 tasks.
     """
     a, b, c, _, _ = abcde
     a_ = _to_keys(a, "0123456789")
@@ -114,6 +118,9 @@ def test_nearest_neighbor(abcde, num_Bs, BRootish):
     assert len(groups) == 2
 
     if BRootish:
+        # As soon as a TG has five or more dependencies, we are no longer
+        # considering it rootish.
+        assert num_Bs == 5
         assert not groups["a"].rootish
         assert groups["b"].rootish
     else: