Hi,
I've recently written a threaded client server application using a custom message protocol. Each message is 49 bytes long and needs to arrive at the client as soon as possible.
In the current app I send() each message as soon as all 49 bytes are available, I also recv(49) bytes at the client side (buffering as necessary). The threaded application thus always reads 49 bytes from the network buffers as soon as it is possible to do so. This application consumes about 9% cpu time at full throttle.
Because I already use the twisted.enterprise adbapi on the client side I decided to get rid of all the client threads and use twisted for the TCP stuff as well. I've written a small test script to determine basic loads and performance:
----------------- #! /usr/bin/env python
import psyco psyco.full()
import array import sys
from twisted.internet.protocol import Protocol,ReconnectingClientFactory from twisted.internet import reactor
f = open("wakka", "w")
class TRAUReceiver(Protocol):
def dataReceived(self, data): print len(data) f.write(data) # f.flush()
def connectionMade(self): """ * connection were made, send signon"""
print "Signing on...", signon = array.array('B', [ord('T'), 1 ,255, 1, 255]) self.transport.write(signon.tostring()) print "Done"
class TRAUClientFactory(ReconnectingClientFactory):
def startedConnecting(self, connector): print 'Started to connect.'
def buildProtocol(self, addr): print 'Connected.' print 'Resetting reconnection delay' self.resetDelay() return TRAUReceiver()
def clientConnectionLost(self, connector, reason): print 'Lost connection. Reason:', reason
ReconnectingClientFactory.clientConnectionLost(self, connector, reason)
def clientConnectionFailed(self, connector, reason): print 'Connection failed. Reason:', reason ReconnectingClientFactory.clientConnectionFailed(self, connector, reason)
reactor.connectTCP('localhost', 55555, TRAUClientFactory()) reactor.run() ------------------
Connecting the above to the server yielded somewhat surprising results:
The length of the dataReceived() data between runs varies between 49 and multiples of 49 bytes. I understand this (I think) as the ethernet packet length sweet spot is about 1.3kB and Protocol is propably optimized arround that. This does imply that I need a more elaborate frame caching scheme on the client side than the one I currently have, as another socket connection signals if messages from the current connection must be stored or discarded.
Surprisingly, the script data length is reported as continuous 49 bytes at leas 4/10 times it's run, on other runs the print len(data) line looks something like this:
49 12691 49 7938 49
Each second dataReceived() callback is 49 bytes, the rest multiples of 49.
When the script reports all frames as 49; cpu consumption is ~33%, when staggered it's ~4%. The dataReceived() call seems overly expensive when compared with the 'raw' synchronous recv().
Paradoxically, padding the 49 byte message with 1000 pad bytes improves the script's performance 8 times; due to the decrease in dataReceived() calls probably.
So, 1) is there a way to force dataReceived() to return when a certain data length has been received ? 2) Why is dataReceived() so expensive (if it is) ? 3) Is Protocol the correct tree or are there other ways to handle small time sensitive messages in twisted.
Apologies for the longish post.
Regards Thys