configurable hashring replicas

as a trade off between less load on coordination backend vs higher
worker utilisation.

Change-Id: I679e79edf857be3660a80e54e039ded60e8431b5

gnocchi/cli.py

@@ -186,10 +186,10 @@ class MetricProcessor(MetricProcessBase):
             if (not self._tasks or
                     self.group_state != self.partitioner.ring.nodes):
                 self.group_state = self.partitioner.ring.nodes.copy()
-                # TODO(gordc): make replicas configurable
                 self._tasks = [
                     i for i in six.moves.range(self.store.incoming.NUM_SACKS)
-                    if self.partitioner.belongs_to_self(i, replicas=3)]
+                    if self.partitioner.belongs_to_self(
+                        i, replicas=self.conf.metricd.processing_replicas)]
         finally:
             return self._tasks or self.fallback_tasks
 

gnocchi/opts.py

@@ -91,7 +91,13 @@ def list_opts():
                             "leave system (in seconds). A shorter rate, may "
                             "improve rebalancing but create more coordination "
                             "load"),
-
+            cfg.IntOpt('processing_replicas',
+                       default=3,
+                       min=1,
+                       help="Number of workers that share a task. A higher "
+                       "value may improve worker utilization but may also "
+                       "increase load on coordination backend. Value is "
+                       "capped by number of workers globally."),
         )),
         ("api", (
             cfg.StrOpt('paste_config',