[En-Nut-Discussion] TCP Sockets: Slower than expected response time.

[Ulrich Hertlein]

Josh Arnold wrote:
> We have an Ethernut implementation that talks to our own PC hosted GUI.  We
> have been battling with slower than expected response times when
> passing/processing packets back to the GUI.  The packets are typically small
> but lots of them.
> 
> Our setup is NutOs running on an ATmega128 with a DM9000E controller.
> 
> Here is an example data exchange.
> GUI                              Server(Ethernut)
> Send ASCII cmd------->
>     <--------------------ACK  cmd received(<1ms)   Ok, this is good
>     <--------------------PSH Data in response to CMD (99ms  this too is
> slow)
> ACK Data received------>(200 ms after data received from server)

Are you sending your response packets in one chunk (one write call) or a
number of smaller ones? In the latter case it's possible you're hit by
'Nagles algorithm' http://en.wikipedia.org/wiki/Nagle's_Algorithm

It basically queues TCP packets for a small time until either a timeout
has expired or enough data is queued.

To work around this you could either collect your response data into a
single write or disable the algorithm using setsockopt TCP_NODELAY.

Hope this helps,
/Ulrich

-- 
Ulrich Hertlein | Software Development

ART+COM AG
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