Add an option for WSGI pool size

Neutron currently hardcodes the number of
greenlets used to process requests in a process to 1000.
As detailed in
http://lists.openstack.org/pipermail/openstack-dev/2015-December/082717.html

this can cause requests to wait within one process
for available database connection while other processes
remain available.

By adding a wsgi_default_pool_size option functionally
identical to that of Nova, we can lower the number of
greenlets per process to be more in line with a typical
max database connection pool size.

DocImpact: a previously unused configuration value
           wsgi_default_pool_size is now used to affect
           the number of greenlets used by the server. The
           default number of greenlets also changes from 1000
           to 100.
Change-Id: I94cd2f9262e0f330cf006b40bb3c0071086e5d71
(cherry picked from commit 9d573387f1)

neutron/wsgi.py

@@ -113,10 +113,10 @@ class WorkerService(worker.NeutronWorker):
 class Server(object):
     """Server class to manage multiple WSGI sockets and applications."""
 
-    def __init__(self, name, num_threads=1000, disable_ssl=False):
+    def __init__(self, name, num_threads=None, disable_ssl=False):
         # Raise the default from 8192 to accommodate large tokens
         eventlet.wsgi.MAX_HEADER_LINE = CONF.max_header_line
-        self.num_threads = num_threads
+        self.num_threads = num_threads or CONF.wsgi_default_pool_size
         self.disable_ssl = disable_ssl
         # Pool for a greenthread in which wsgi server will be running
         self.pool = eventlet.GreenPool(1)

releasenotes/notes/config-wsgi-pool-size-a4c06753b79fee6d.yaml

@@ -0,0 +1,12 @@
+---
+prelude: >
+    Support configuration of greenthreads pool for WSGI.
+other:
+  - Operators may want to tune the ``max_overflow`` and
+    ``wsgi_default_pool_size`` configuration options according
+    to the investigations outlined in this `mailing list post
+    <http://lists.openstack.org/pipermail/openstack-dev/2015-December/082717.html>`_.
+    The default value of ``wsgi_default_pool_size`` inherits from
+    that of oslo.config, which is currently 100.  This is
+    a change in default from the previous Neutron-specific
+    value of 1000.