add copy_threshold to limit use of zero-copy to sufficiently large messages

threshold is triggered on the largest part of the message, and copy is applied to all parts, to avoid split-sends.

IPython/zmq/session.py

@@ -49,7 +49,7 b' from IPython.utils.importstring import import_item'
 from IPython.utils.jsonutil import extract_dates, squash_dates, date_default
 from IPython.utils.py3compat import str_to_bytes
 from IPython.utils.traitlets import (CBytes, Unicode, Bool, Any, Instance, Set,
-                                        DottedObjectName, CUnicode, Dict)
+                                        DottedObjectName, CUnicode, Dict, Int)
 
 #-----------------------------------------------------------------------------
 # utility functions
@@ -84,8 +84,9 b' pickle_unpacker = pickle.loads'
 default_packer = json_packer
 default_unpacker = json_unpacker
 
-DELIM=b"<IDS|MSG>"
-
+DELIM = b"<IDS|MSG>"
+# singleton dummy tracker, which will always report as done
+DONE = zmq.MessageTracker()
 
 #-----------------------------------------------------------------------------
 # Mixin tools for apps that use Sessions
@@ -329,6 +330,9 b' class Session(Configurable):'
         # unpacker is not checked - it is assumed to be
         if not callable(new):
             raise TypeError("unpacker must be callable, not %s"%type(new))
+    
+    copy_threshold = Int(2**12, config=True,
+        help="Threshold (in bytes) beyond which a buffer should be sent without copying.")
 
     def __init__(self, **kwargs):
         """create a Session object
@@ -544,11 +548,6 b' class Session(Configurable):'
         -------
         msg : dict
             The constructed message.
-        (msg,tracker) : (dict, MessageTracker)
-            if track=True, then a 2-tuple will be returned,
-            the first element being the constructed
-            message, and the second being the MessageTracker
-
         """
 
         if not isinstance(stream, (zmq.Socket, ZMQStream)):
@@ -566,25 +565,18 b' class Session(Configurable):'
 
         buffers = [] if buffers is None else buffers
         to_send = self.serialize(msg, ident)
-        flag = 0
-        if buffers:
-            flag = zmq.SNDMORE
-            _track = False
+        to_send.extend(buffers)
+        longest = max([ len(s) for s in to_send ])
+        copy = (longest > self.copy_threshold)
+        
+        if buffers and track and not copy:
+            # only really track when we are doing zero-copy buffers
+            tracker = stream.send_multipart(to_send, copy=False, track=True)
         else:
-            _track=track
-        if track:
-            tracker = stream.send_multipart(to_send, flag, copy=False, track=_track)
-        else:
-            tracker = stream.send_multipart(to_send, flag, copy=False)
-        for b in buffers[:-1]:
-            stream.send(b, flag, copy=False)
-        if buffers:
-            if track:
-                tracker = stream.send(buffers[-1], copy=False, track=track)
-            else:
-                tracker = stream.send(buffers[-1], copy=False)
+            # use dummy tracker, which will be done immediately
+            tracker = DONE
+            stream.send_multipart(to_send, copy=copy)
 
-        # omsg = Message(msg)
         if self.debug:
             pprint.pprint(msg)
             pprint.pprint(to_send)