Cap the number of worker threads

Users can configure the number of worker threads however when it's
not specified the calculated number of workers can get too large on
hosts with a large number of CPUs.

Change-Id: Ie4fae6e015a58399cb8656e88d6b9aa514ecaa5d
(cherry picked from commit 3d16c31779)

defaults/main.yml

@@ -165,6 +165,10 @@ heat_watch_server_url: "{{ heat_watch_server_uri }}"
 # heat_engine_workers: 4
 # heat_api_workers: 4
 
+## Cap the maximum number of threads / workers when a user value is unspecified.
+heat_api_threads_max: 16
+heat_api_threads: "{{ [[ansible_processor_vcpus|default(2) // 2, 1] | max, heat_api_threads_max] | min }}"
+
 heat_service_in_ldap: false
 
 ## Plugin dirs

releasenotes/notes/capping_heat_workers-13791c456e59277d.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - Capping the default value for the variables
+    ``heat_api_workers`` and ``heat_engine_workers`` to 16 when the user
+    doesn't configure these variables. Default value is half the number of
+    vCPUs available on the machine with a capping value of 16.

templates/heat.conf.j2

@@ -1,8 +1,5 @@
 # {{ ansible_managed }}
 
-{% set _api_threads = ansible_processor_vcpus|default(2) // 2 %}
-{% set api_threads = _api_threads if _api_threads > 0 else 1 %}
-
 [DEFAULT]
 # Disable stderr logging
 use_stderr = False
@@ -24,7 +21,7 @@ trusts_delegated_roles = {{ heat_trusts_delegated_roles | join(',') }}
 
 auth_encryption_key = {{ heat_auth_encryption_key }}
 
-num_engine_workers = {{ heat_engine_workers | default(api_threads) }}
+num_engine_workers = {{ heat_engine_workers | default(heat_api_threads) }}
 
 ## RPC Backend
 rpc_backend = {{ heat_rpc_backend }}
@@ -55,7 +52,7 @@ auth_uri = {{ keystone_service_internalurl }}
 
 [heat_api]
 bind_port = {{ heat_service_port }}
-workers = {{ heat_api_workers | default(api_threads) }}
+workers = {{ heat_api_workers | default(heat_api_threads) }}
 
 [heat_api_cfn]
 bind_port = {{ heat_cfn_service_port }}