Re: [Discuss-gnuradio] UHD:SingleUSRP and buffer allocation policy

[Josh Blum]

> >
> Hard to get objective numbers, but it seems like there's no difference
> whether I have the audio sub-graph enabled or
>    disabled.
>
> The absolute minimum latency I can observe is about 1 second or so, with
> an input recv_buff_size=2500.
>
> I know that there's buffering in the USRP2, but not that much--32K?
>   Less?  More?  Even so, at my sample rate, 32K is
>    only about 20ms of latency.

The host cannot back-pressure the USRP2, and the buffering in the USRP2 
itself if negligible. So, all of the backing-up that is going to happen 
occurs in the host when you dont read data out fast enough.

The UHD-USRP2 receive path has two buffers that fill when calls to 
recv() dont keep up. One is the socket buffer in the kernel, and the 
other is a UHD allocated buffer that hangs onto frames from the 
socket->receive() call. Both of those drop when filled, so any further 
buffering/latency you are seeing is happening in gnuradio.

You can manipulate the socket buffer size and the number of frames: 
http://www.ettus.com/uhd_docs/manual/html/transport.html#transport-parameters

-Josh