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: I09433117941dcd543775d5c347cb03912c21475c

defaults/main.yml

@@ -125,6 +125,10 @@ horizon_disallow_iframe_embed: True
 # horizon_wsgi_processes: 4
 # horizon_wsgi_threads: 4
 
+## Cap the maximun number of threads / workers when a user value is unspecified.
+horizon_wsgi_threads_max: 16
+horizon_wsgi_threads: "{{ [[ansible_processor_vcpus|default(2) // 2, 1] | max, horizon_wsgi_threads_max] | min }}"
+
 ## Horizon SSL
 horizon_ssl_cert: /etc/ssl/certs/horizon.pem
 horizon_ssl_key: /etc/ssl/private/horizon.key

releasenotes/notes/capping_horizon_workers-29ecc4893bcc3a4b.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - Capping the default value for the variables
+    ``horizon_wsgi_processes`` and ``horizon_wsgi_threads`` 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/openstack_dashboard.conf.j2

@@ -1,8 +1,5 @@
 # {{ ansible_managed }}
 
-{% set _wsgi_threads = ansible_processor_vcpus | default(2) // 2 %}
-{% set wsgi_threads = _wsgi_threads if _wsgi_threads > 0 else 1 %}
-
 {% if not horizon_external_ssl | bool %}
 <VirtualHost *:80>
     ServerName {{ horizon_server_name }}
@@ -36,7 +33,7 @@
 {% endif %}
 
     WSGIScriptAlias / {{ horizon_lib_wsgi_file }}
-    WSGIDaemonProcess horizon user={{ horizon_system_user_name }} group={{ horizon_system_group_name }} processes={{ horizon_wsgi_processes | default(wsgi_threads) }} threads={{ horizon_wsgi_threads | default(wsgi_threads) }} python-path={{ horizon_bin | dirname }}/lib/python2.7/site-packages
+    WSGIDaemonProcess horizon user={{ horizon_system_user_name }} group={{ horizon_system_group_name }} processes={{ horizon_wsgi_processes | default(horizon_wsgi_threads) }} threads={{ horizon_wsgi_threads }} python-path={{ horizon_bin | dirname }}/lib/python2.7/site-packages
 
     WSGIProcessGroup horizon
     WSGIApplicationGroup horizon