Re: [Discuss-gnuradio] Dependency of RFNoC FFT size on Ethernet MTU

[Marcus D. Leech]

On 02/16/2016 10:56 AM, William Healey wrote:
> Hello,
>
> I am trying to make a simple FFT flow chart using a RFNoC FFT block 
> and a signal source as input. I am finding that the maximum FFT size I 
> can use is limited by the MTU of my Ethernet connection. While 
> connected to the 1GB Ethernet I had a MTU of 1500 and was able to use 
> an FFT size of 1024 but if I tried to use 2048 I would get a timeout 
> on chan0 error message, similar to this case 
> http://comments.gmane.org/gmane.comp.hardware.usrp.e100/13915 . I was 
> able to increase the FFT size to 1024 because I stayed in the RFNoc 
> layer to display data instead of moving back up to the host. If I move 
> up to the host layer then I am limited to FFT size of 512 like the 
> previous case.
>
> I switched to a 10GB connection which supports a MTU of 9000. This 
> gave me a new max FFT size of 2048 if I stay in the RFNoC layer and 
> 1024 if I move back up to the host layer. To do this I had to edit the 
> code for the RFNoC block like in this case 
> http://comments.gmane.org/gmane.comp.hardware.usrp.e100/16564. In that 
> example it is mentioned that it may be possible to break up larger 
> FFTs into multiple packets and recombine them.
>
> I am wondering if there is a way to increase the FFT size past 2048 
> with a limited MTU. It aslo seems that the max packet size for the 
> USRP is 8000 so increasing the MTU further may be of little use. 
> FInally I am wondering if it is possible to have an FFT size up to 
> 8192 and if not, will it be in the future?
>
> I have attached my flowchart below
The RFNOC cross-bar exchanges VITA frames, from what I understand. These 
then end up unmolested going out the ethernet port.  I think
  that there are long-term plans to have some kind of fragmentation 
scheme, but it isn't implemented.   On 1GiGe/10GiGe, the ethernet
  standards limit you to jumbo frames of no more than 9000 bytes, which 
means FFTs of no more than 2048 bins  (4 bytes per FFT bin).
  MANY host ethernet cards either don't support jumbo frames, or only 
support a limited size of jumbo frames, and not up to 9000 bytes.