Optimization of waiting subprocesses in ProcessLauncher

Because of eventlet bug "eventlet.green override of os.waitpid
returns immediately in all cases"(
https://bitbucket.org/eventlet/eventlet/issue/92/eventletgreen-override-of-oswaitpid)
in scope of fixing openstack bug "Excessive CPU usage in ProcessLauncher()'s wait loop"
(https://bugs.launchpad.net/oslo-incubator/+bug/109534) additional sleep was added.
Since this bug is resolved and openstack depends on eventlet >=0.16
excessive sleep was removed.

Change-Id: Ief30758ff429b4adc2265a8ed7ae34c5b8c60a34

openstack/common/service.py

@@ -199,16 +199,12 @@ class ServiceWrapper(object):
 
 
 class ProcessLauncher(object):
-    def __init__(self, wait_interval=0.01):
-        """Constructor.
+    def __init__(self):
+        """Constructor."""
 
-        :param wait_interval: The interval to sleep for between checks
-                              of child process exit.
-        """
         self.children = {}
         self.sigcaught = None
         self.running = True
-        self.wait_interval = wait_interval
         rfd, self.writepipe = os.pipe()
         self.readpipe = eventlet.greenio.GreenPipe(rfd, 'r')
         self.handle_signal()
@@ -333,8 +329,8 @@ class ProcessLauncher(object):
 
     def _wait_child(self):
         try:
-            # Don't block if no child processes have exited
-            pid, status = os.waitpid(0, os.WNOHANG)
+            # Block while any of child processes have exited
+            pid, status = os.waitpid(0, 0)
             if not pid:
                 return None
         except OSError as exc:
@@ -363,10 +359,6 @@ class ProcessLauncher(object):
         while self.running:
             wrap = self._wait_child()
             if not wrap:
-                # Yield to other threads if no children have exited
-                # Sleep for a short time to avoid excessive CPU usage
-                # (see bug #1095346)
-                eventlet.greenthread.sleep(self.wait_interval)
                 continue
             while self.running and len(wrap.children) < wrap.workers:
                 self._start_child(wrap)