Re: [lwip-users] Tx queue filling up

[address@hidden]

Jeff Barber wrote:
> Now 256 happens to correlate to the size of the Tx and Rx ring buffers
> in my driver so that was the obvious place to look.  I notice that
> sometimes my driver's linkoutput function is being called when its Tx
> queue is full.  I'm thinking that's the proximate cause of the
> problem.  However, shouldn't the TCP_SND_QUEUELEN (32) and TCP_SND_BUF
> (8 * MSS) values be a limit to the maximum number of "outstanding"
> pbufs?  I did turn on TCP_QLEN_DEBUG and according to that info, I
> never get a queue len > 8.  So why do I end up with so many packets
> "in flight"?
>   
That's a good question. The queue len doesn't grow because SND_BUF 
limits it (assuming you transmitted 8 full mss-sized segments).

When your driver runs out of buffers, did you check these are only 
packets from one tcp connection? Maybe there are other packets 
transmitted by your board?
> And if I'm misunderstanding, what is the intended feedback mechanism
> from the driver?  tcp_output always seems to ignore the return value
> of ip_output.  Hence, if I understand it correctly, that means an
> attempt to send while the Tx queue is full is treated exactly the same
> as if the packet was dropped on the wire: i.e. it will rely on the
> retransmission process to recover.
>   
Unfortunately, this really is the case currently. A solution for this 
isto just delay the lwip sending task until there's room in your MAC's 
Tx queue. Unfortunately, this also suppresses processing Rx packets 
while waiting, but at least other tasks can run. The problem is at one 
point you *have* to slow down the CPU if it tries to send faster than 
the wire supports. Any input on how this can best be solved in the lwIP 
code is of course welcome!

Simon