Add some tunable parameters for nova-scheduler

Since placement service was introduced, there have been several
conditions where instances are not well distributed among multiple
compute nodes. For example with the default configurations if a user
creates multiple instances concurrently, these instances might be
packed into a single compute node because placement returns the same
results until any instance is actually launched at compute nodes.

This change introdices two tunable parameters about nova-scheduler
so that we can implement better distribution of nova instances in some
usecases.

Change-Id: Id488db4bd60b1c35704e24a0a85d61853bfd7038

deployment/nova/nova-scheduler-container-puppet.yaml

@@ -118,6 +118,16 @@ parameters:
       aggregates to be reported in placement, so only hosts within the asked
       aggregates would be accepted.
     type: boolean
+  NovaSchedulerHostSubsetSize:
+    default: 1
+    description: >
+      Size of subset of best hosts selected by scheduler.
+    type: number
+  NovaSchedulerShuffleBestSameWeighedHosts:
+    default: false
+    description: >
+      Enable spreading the instances between hosts with the same best weight.
+    type: boolean
   # DEPRECATED: the following options are deprecated and are currently maintained
   # for backwards compatibility. They will be removed in future release.
   NovaSchedulerDefaultFilters:
@@ -206,6 +216,8 @@ outputs:
               - {get_param: NovaSchedulerDefaultFilters}
               - {get_param: NovaSchedulerEnabledFilters}
             nova::scheduler::filter::scheduler_max_attempts: {get_param: NovaSchedulerMaxAttempts}
+            nova::scheduler::filter::scheduler_host_subset_size: {get_param: NovaSchedulerHostSubsetSize}
+            nova::scheduler::filter::shuffle_best_same_weighed_hosts: {get_param: NovaSchedulerShuffleBestSameWeighedHosts}
             nova::scheduler::discover_hosts_in_cells_interval: {get_param: NovaSchedulerDiscoverHostsInCellsInterval}
             nova::scheduler::query_placement_for_image_type_support: {get_param: NovaSchedulerQueryImageType}
             nova::scheduler::limit_tenants_to_placement_aggregate: {get_param: NovaSchedulerLimitTenantsToPlacementAggregate}

releasenotes/notes/nova-scheduler-tunables-8c1dbab10b289480.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    Add the following parameters to tune the behavior of nova-scheduler to
+    achieve better distribution of instances.
+
+    - ``NovaSchedulerHostSubsetSize``
+    - ``NovaSchedulerShuffleBestSameWeighedHosts``