TST: Add a new test (untested, sorry) and a brief comment

python/raft-dask/raft_dask/common/comms.py

@@ -693,6 +693,8 @@ def _func_worker_ranks(client, workers):
     For each worker connected to the client, compute a global rank which takes
     into account the NVML device index and the worker IP
     (group workers on same host and order by NVML device).
+    Note that the reason for sorting was nvbug 4149999 and is presumably
+    fixed afterNCCL 2.19.3.
 
     Parameters
     ----------

python/raft-dask/raft_dask/test/test_comms.py

@@ -246,6 +246,17 @@ def test_collectives(client, func, root_location):
             cb.destroy()
 
 
+@pytest.mark.nccl
+@pytest.mark.parametrize("subset", [-1, 1, slice(None, None, -2)])
+def test_comm_init_worker_subset(client, subset):
+    # Basic test that initializing a subset of workers is fine
+    cb = Comms(comms_p2p=True, verbose=True)
+
+    workers = list(client.scheduler_info()["workers"].keys())
+    workers = workers[subset]
+    cb.init(workers=workers)
+
+
 @pytest.mark.nccl
 def test_comm_split(client):