Merge pull request #1312 from minrk/hb

two small heartbeat changes:

* flush outgoing heartbeats (notebook and parallel):
  - Prevents mismatch between heartbeat timer and actual heartbeat send/recv events.

* relax default heartbeat period in IPython.parallel to 3s from 1s
  - Matches heartbeats elsewhere.  3s should be extremely conservative, as most heartbeat responses are O(1ms) unless there's a bug in our code (like the above flush and previously-discovered GIL issues).

closes #1304

IPython/frontend/html/notebook/handlers.py

@@ -481,6 +481,8 @@ def ping_or_dead():
                 if self._kernel_alive:
                     self._kernel_alive = False
                     self.hb_stream.send(b'ping')
+                    # flush stream to force immediate socket send
+                    self.hb_stream.flush()
                 else:
                     try:
                         callback()

IPython/parallel/controller/heartmonitor.py

@@ -64,7 +64,7 @@ class HeartMonitor(LoggingConfigurable):
     pongstream: an XREP stream
     period: the period of the heartbeat in milliseconds"""
 
-    period = Integer(1000, config=True,
+    period = Integer(3000, config=True,
         help='The frequency at which the Hub pings the engines for heartbeats '
         '(in ms)',
     )
@@ -124,6 +124,8 @@ def beat(self):
         # print self.on_probation, self.hearts
         # self.log.debug("heartbeat::beat %.3f, %i beating hearts", self.lifetime, len(self.hearts))
         self.pingstream.send(asbytes(str(self.lifetime)))
+        # flush stream to force immediate socket send
+        self.pingstream.flush()
 
     def handle_new_heart(self, heart):
         if self._new_handlers: