Fix default RPC worker count

The help for the rpc_workers config option is:

    Number of RPC worker processes for service.  If not specified, the
    default is equal to half the number of API workers.

However, this does not accurately describe the current behaviour,
which is to default to half the _default_ number of API workers.  This
can make a big difference; for example on a 256-CPU machine with 256GB
of RAM which has api_workers configured to 8 but rpc_workers not
configured to anything, this will result in 64 RPC workers, which is 8
for every API worker!

Therefore tweak the default to rely on the actual value of
api_workers, which may be different than the default value.

Change-Id: I26115932ef4775f157297be1637ee26a4fca4666
Related-Bug: #1838688
Closes-Bug: #1838689

neutron/service.py

@@ -176,7 +176,7 @@ def _get_rpc_workers(plugin=None):
     workers = cfg.CONF.rpc_workers
     if workers is None:
         # By default, half as many rpc workers as api workers
-        workers = int(_get_worker_count() / 2)
+        workers = int(_get_api_workers() / 2)
     if workers < 1:
         workers = 1
 

neutron/tests/unit/test_service.py

@@ -61,9 +61,13 @@ class TestRunRpcWorkers(base.BaseTestCase):
     def test_rpc_workers_zero(self):
         self._test_rpc_workers(0, 1)
 
-    def test_rpc_workers_default(self):
+    def test_rpc_workers_default_api_workers_default(self):
         self._test_rpc_workers(None, int(self.worker_count / 2))
 
+    def test_rpc_workers_default_api_workers_set(self):
+        cfg.CONF.set_override('api_workers', 18)
+        self._test_rpc_workers(None, 9)
+
     def test_rpc_workers_defined(self):
         self._test_rpc_workers(42, 42)