diff --git a/src/python/grpcio_tests/tests/_runner.py b/src/python/grpcio_tests/tests/_runner.py
index 6411ffa8981..804b1a6e331 100644
--- a/src/python/grpcio_tests/tests/_runner.py
+++ b/src/python/grpcio_tests/tests/_runner.py
@@ -117,7 +117,7 @@ class AugmentedCase(collections.namedtuple('AugmentedCase', ['case', 'id'])):
 
 class Runner(object):
 
-    def __init__(self, dedicated_threads=True):
+    def __init__(self, dedicated_threads=False):
         """Constructs the Runner object.
 
         Args:
@@ -202,11 +202,13 @@ class Runner(object):
                     augmented_case.case.id()))
                 sys.stdout.flush()
                 if self._dedicated_threads:
+                    # (Deprecated) Spawns dedicated thread for each test case.
                     case_thread = threading.Thread(
                         target=augmented_case.case.run, args=(result,))
                     try:
                         with stdout_pipe, stderr_pipe:
                             case_thread.start()
+                            # If the thread is exited unexpected, stop testing.
                             while case_thread.is_alive():
                                 check_kill_self()
                                 time.sleep(0)
@@ -214,6 +216,7 @@ class Runner(object):
                     except:  # pylint: disable=try-except-raise
                         # re-raise the exception after forcing the with-block to end
                         raise
+                    # Records the result of the test case run.
                     result.set_output(augmented_case.case, stdout_pipe.output(),
                                       stderr_pipe.output())
                     sys.stdout.write(result_out.getvalue())
@@ -221,6 +224,7 @@ class Runner(object):
                     result_out.truncate(0)
                     check_kill_self()
                 else:
+                    # Donates current thread to test case execution.
                     augmented_case.case.run(result)
         result.stopTestRun()
         stdout_pipe.close()
diff --git a/src/python/grpcio_tests/tests_aio/unit/_test_server.py b/src/python/grpcio_tests/tests_aio/unit/_test_server.py
index dd2d7a98ed0..5f3661f42cf 100644
--- a/src/python/grpcio_tests/tests_aio/unit/_test_server.py
+++ b/src/python/grpcio_tests/tests_aio/unit/_test_server.py
@@ -24,7 +24,7 @@ from src.proto.grpc.testing import test_pb2_grpc
 from tests.unit.framework.common import test_constants
 
 
-class TestServiceServicer(test_pb2_grpc.TestServiceServicer):
+class _TestServiceServicer(test_pb2_grpc.TestServiceServicer):
 
     async def UnaryCall(self, request, context):
         return messages_pb2.SimpleResponse()
@@ -36,7 +36,7 @@ class TestServiceServicer(test_pb2_grpc.TestServiceServicer):
 
 async def start_test_server():
     server = aio.server(options=(('grpc.so_reuseport', 0),))
-    test_pb2_grpc.add_TestServiceServicer_to_server(TestServiceServicer(),
+    test_pb2_grpc.add_TestServiceServicer_to_server(_TestServiceServicer(),
                                                     server)
     port = server.add_insecure_port('[::]:0')
     await server.start()