diff --git a/mitogen/core.py b/mitogen/core.py
index 58289553..7c2670ef 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -370,6 +370,8 @@ def set_block(fd):
     fcntl.fcntl(fd, fcntl.F_SETFL, flags & ~os.O_NONBLOCK)
 
 
+_IO_ERRORS = (select.error, OSError, IOError)
+
 def io_op(func, *args):
     """Wrap `func(*args)` that may raise :class:`select.error`,
     :class:`IOError`, or :class:`OSError`, trapping UNIX error codes relating
@@ -393,7 +395,7 @@ def io_op(func, *args):
     while True:
         try:
             return func(*args), False
-        except (select.error, OSError, IOError):
+        except _IO_ERRORS:
             e = sys.exc_info()[1]
             _vv and IOLOG.debug('io_op(%r) -> OSError: %s', func, e)
             if e.args[0] == errno.EINTR:
@@ -1352,6 +1354,7 @@ class Stream(BasicStream):
         self.name = u'default'
         self.sent_modules = set(['mitogen', 'mitogen.core'])
         self.construct(**kwargs)
+        self._lock = threading.Lock()
         self._input_buf = collections.deque()
         self._output_buf = collections.deque()
         self._input_buf_len = 0
@@ -1462,20 +1465,40 @@ class Stream(BasicStream):
         if not self._output_buf:
             broker._stop_transmit(self)
 
-    def _send(self, msg):
-        _vv and IOLOG.debug('%r._send(%r)', self, msg)
+    def send(self, msg):
+        """Send `data` to `handle`, and tell the broker we have output. May
+        be called from any thread."""
+        _vv and IOLOG.debug('%r.send(%r)', self, msg)
         pkt = struct.pack(self.HEADER_FMT, msg.dst_id, msg.src_id,
                           msg.auth_id, msg.handle, msg.reply_to or 0,
                           len(msg.data)) + msg.data
-        if not self._output_buf_len:
-            self._router.broker._start_transmit(self)
-        self._output_buf.append(pkt)
-        self._output_buf_len += len(pkt)
+        pktlen = len(pkt)
 
-    def send(self, msg):
-        """Send `data` to `handle`, and tell the broker we have output. May
-        be called from any thread."""
-        self._router.broker.defer(self._send, msg)
+        self._lock.acquire()
+        try:
+            if self._output_buf_len:
+                self._output_buf.append(pkt)
+                self._output_buf_len += pktlen
+                return
+
+            written = None
+            try:
+                written = self.transmit_side.write(pkt)
+            except _IO_ERRORS:
+                e = sys.exc_info()[1]
+                if e.args[0] not in (errno.EWOULDBLOCK, errno.EAGAIN):
+                    raise
+
+            if not written:
+                self._output_buf.append(pkt)
+                self._output_buf_len += pktlen
+            elif written != pktlen:
+                self._output_buf.append(buffer(pkt, written))
+                self._output_buf_len += pktlen - written
+                broker = self._router.broker
+                broker.defer(broker._start_transmit, self)
+        finally:
+            self._lock.release()
 
     def on_shutdown(self, broker):
         """Override BasicStream behaviour of immediately disconnecting."""
@@ -1778,8 +1801,6 @@ class Latch(object):
             self._cls_all_sockets.extend((rsock, wsock))
             return rsock, wsock
 
-    COOKIE_SIZE = 33
-
     def _make_cookie(self):
         """
         Return a 33-byte string encoding the ID of the instance and the current
@@ -1787,7 +1808,12 @@ class Latch(object):
         the FD, and buggy internal FD sharing.
         """
         ident = threading.currentThread().ident
-        return b(u'%016x-%016x' % (int(id(self)), ident))
+        return b(
+            (u'%-8d-%-16x-%-16x' % (os.getpid(), int(id(self)), ident))
+            .replace(' ', '-')
+        )
+
+    COOKIE_SIZE = len(_make_cookie(None))
 
     def get(self, timeout=None, block=True):
         """
@@ -2360,7 +2386,7 @@ class Router(object):
                 in_stream.remote_id, out_stream.remote_id)
             return
 
-        out_stream._send(msg)
+        out_stream.send(msg)
 
     def route(self, msg):
         """
@@ -2372,7 +2398,7 @@ class Router(object):
 
         This may be called from any thread.
         """
-        self.broker.defer(self._async_route, msg)
+        self._async_route(msg)
 
 
 class Broker(object):
@@ -2498,7 +2524,8 @@ class Broker(object):
 
     def _broker_exit(self):
         for _, (side, _) in self.poller.readers + self.poller.writers:
-            LOG.error('_broker_main() force disconnecting %r', side)
+            if side.keep_alive:
+                LOG.error('_broker_main() force disconnecting %r', side)
             side.stream.on_disconnect(self)
 
         self.poller.close()
diff --git a/mitogen/parent.py b/mitogen/parent.py
index 04f7784e..3e961566 100644
--- a/mitogen/parent.py
+++ b/mitogen/parent.py
@@ -995,7 +995,7 @@ class Stream(mitogen.core.Stream):
     def on_shutdown(self, broker):
         """Request the slave gracefully shut itself down."""
         LOG.debug('%r closing CALL_FUNCTION channel', self)
-        self._send(
+        self.send(
             mitogen.core.Message(
                 src_id=mitogen.context_id,
                 dst_id=self.remote_id,
@@ -1962,7 +1962,7 @@ class ModuleForwarder(object):
                   self, fullname, context_id, stream.remote_id)
         self._send_module_and_related(stream, fullname)
         if stream.remote_id != context_id:
-            stream._send(
+            stream.send(
                 mitogen.core.Message(
                     data=msg.data,
                     handle=mitogen.core.FORWARD_MODULE,