issue #627: reduce the default pool size in a child to 2.

Ansible has no blocking services running, or really any service that
would have an outsized benefit from multiple IO waiters. Probably we
only need 1, but let's start with 2 just in case.

docs/changelog.rst

@@ -222,6 +222,10 @@ Core Library
   to a function call, where the reply might be dropped due to exceeding the
   maximum configured message size.
 
+* :gh:issue:`624`: the number of threads used for a child's auto-started thread
+  pool has been reduced from 16 to 2. This may drop to 1 in future, and become
+  configurable via a :class:`Router` option.
+
 * :gh:commit:`a5536c35`: avoid quadratic
   buffer management when logging lines received from a child's redirected
   standard IO.

mitogen/service.py

@@ -55,7 +55,6 @@ except NameError:
 
 LOG = logging.getLogger(__name__)
 
-DEFAULT_POOL_SIZE = 16
 _pool = None
 _pool_pid = None
 #: Serialize pool construction.
@@ -88,7 +87,7 @@ def get_or_create_pool(size=None, router=None):
                 _pool = Pool(
                     router,
                     services=[],
-                    size=size or DEFAULT_POOL_SIZE,
+                    size=size or 2,
                     overwrite=True,
                     recv=mitogen.core.Dispatcher._service_recv,
                 )