Fixed high CPU usage of XREQ channel.

Previously, the XReqSocketChannel was consuming nearly 100% CPU.
This was caused by the ioloop polling for POLLIN|POLLOUT|POLLERR
always. Now we are only polling for POLLOUT when there is a
message waiting to be sent. This seems to have fixed the problem.

IPython/frontend/qt/kernelmanager.py

@@ -3,6 +3,7 b''
 
 # System library imports.
 from PyQt4 import QtCore
+import zmq
 
 # IPython imports.
 from IPython.zmq.kernelmanager import KernelManager, SubSocketChannel, \
@@ -10,6 +11,7 b' from IPython.zmq.kernelmanager import KernelManager, SubSocketChannel, \\'
 from util import MetaQObjectHasTraits
 
 
+
 class QtSubSocketChannel(SubSocketChannel, QtCore.QObject):
 
     # Emitted when any message is received.
@@ -95,6 +97,7 b' class QtXReqSocketChannel(XReqSocketChannel, QtCore.QObject):'
         """ Reimplemented to skip callback handling.
         """
         self.command_queue.put(msg)
+        self.add_io_state(zmq.POLLOUT)
 
 
 class QtRepSocketChannel(RepSocketChannel, QtCore.QObject):

IPython/zmq/kernelmanager.py

@@ -34,6 +34,12 b' class ZmqSocketChannel(Thread):'
     """ The base class for the channels that use ZMQ sockets.
     """
 
+    context = None
+    session = None
+    socket = None
+    ioloop = None
+    iostate = None
+
     def __init__(self, context, session, address=None):
         super(ZmqSocketChannel, self).__init__()
         self.daemon = True
@@ -41,7 +47,6 b' class ZmqSocketChannel(Thread):'
         self.context = context
         self.session = session
         self.address = address
-        self.socket = None
 
     def stop(self):
         """Stop the thread's activity. Returns when the thread terminates.
@@ -73,6 +78,28 b' class ZmqSocketChannel(Thread):'
 
     address = property(get_address, set_adresss)
 
+    def add_io_state(self, state):
+        """Add IO state to the eventloop.
+
+        This is thread safe as it uses the thread safe IOLoop.add_callback.
+        """
+        def add_io_state_callback():
+            if not self.iostate & state:
+                self.iostate = self.iostate | state
+                self.ioloop.update_handler(self.socket, self.iostate)
+        self.ioloop.add_callback(add_io_state_callback)
+
+    def drop_io_state(self, state):
+        """Drop IO state from the eventloop.
+
+        This is thread safe as it uses the thread safe IOLoop.add_callback.
+        """
+        def drop_io_state_callback():
+            if self.iostate & state:
+                self.iostate = self.iostate & (~state)
+                self.ioloop.update_handler(self.socket, self.iostate)
+        self.ioloop.add_callback(drop_io_state_callback)
+
 
 class SubSocketChannel(ZmqSocketChannel):
 
@@ -85,8 +112,9 b' class SubSocketChannel(ZmqSocketChannel):'
         self.socket.setsockopt(zmq.IDENTITY, self.session.session)
         self.socket.connect('tcp://%s:%i' % self.address)
         self.ioloop = ioloop.IOLoop()
+        self.iostate = POLLIN|POLLERR
         self.ioloop.add_handler(self.socket, self._handle_events, 
-                                POLLIN|POLLERR)
+                                self.iostate)
         self.ioloop.start()
 
     def stop(self):
@@ -171,8 +199,9 b' class XReqSocketChannel(ZmqSocketChannel):'
         self.socket.setsockopt(zmq.IDENTITY, self.session.session)
         self.socket.connect('tcp://%s:%i' % self.address)
         self.ioloop = ioloop.IOLoop()
+        self.iostate = POLLERR|POLLIN
         self.ioloop.add_handler(self.socket, self._handle_events, 
-                                POLLIN|POLLOUT|POLLERR)
+                                self.iostate)
         self.ioloop.start()
 
     def stop(self):
@@ -180,7 +209,6 b' class XReqSocketChannel(ZmqSocketChannel):'
         super(XReqSocketChannel, self).stop()
 
     def _handle_events(self, socket, events):
-        # Turn on and off POLLOUT depending on if we have made a request
         if events & POLLERR:
             self._handle_err()
         if events & POLLOUT:
@@ -199,6 +227,8 b' class XReqSocketChannel(ZmqSocketChannel):'
             pass
         else:
             self.socket.send_json(msg)
+        if self.command_queue.empty():
+            self.drop_io_state(POLLOUT)
 
     def _handle_err(self):
         # We don't want to let this go silently, so eventually we should log.
@@ -208,6 +238,7 b' class XReqSocketChannel(ZmqSocketChannel):'
         handler = self._find_handler(msg['msg_type'], callback)
         self.handler_queue.put(handler)
         self.command_queue.put(msg)
+        self.add_io_state(POLLOUT)
 
     def execute(self, code, callback=None):
         # Create class for content/msg creation. Related to, but possibly