Re: [Discuss-gnuradio] Changing Nout_items and Buffer Settings for GNURadio 3.6.2

[Tom Hendrick]

To clarify, I am interested in making my file sampling rate at 125 k S/s and running the USRP at 250 kS/s.  I am using a resampler block in between the file source and usrp sink.  The file source reads data from a fifo file and external modulator code writes to that file.

Thanks, Tom

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From: Tom Hendrick <address@hidden>
To: Tom Rondeau <address@hidden>
Cc: "address@hidden" <address@hidden>
Sent: Wednesday, February 6, 2013 2:54 PM
Subject: Re: [Discuss-gnuradio] Changing Nout_items and Buffer Settings for GNURadio 3.6.2

Hi Tom,
I appreciate your response and information on setting noutput_items for version 3.6.2.  I am not sure if changing the noutput_items or buffer size is going to help at all. 

Basically when the data rate is lower than the transmission rate (purposely to eliminate latency), I'm noticing that the USRP is outputing random gaps in my packet of data during some of the underruns.  Some of the underruns generate a wider gap between packets (this is good) but some of them generate a gap inside the packet data blocks causing problems.  This happens when I am using a 125kHz sampling frequency and the USRP's rate set lower than 1M S/s.  However with the USRP at 1M S/s it works fine.  My goal is to run the USRP at 250k S/s  to reduce processor load.

For the 125k S/s case and lower than 1 M S/s USRP rate, if I increase my packet size by a factor of two, the gaps caused by underruns happen only between packets not inside them.  The problem is the increase in packet size is not desirable for my application.

The behavior I observed makes me think the issue could be fixed with changing the Nout_items or buffer settings.  Any idea if that might be true?
Thank you, Tom

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From: Tom Rondeau <address@hidden>
To: Tom Hendrick <address@hidden>
Cc: "address@hidden" <address@hidden>
Sent: Wednesday, February 6, 2013 2:36 PM
Subject: Re: [Discuss-gnuradio] Changing Nout_items and Buffer Settings for GNURadio 3.6.2

On Wed, Feb 6, 2013 at 1:34 PM, Tom Hendrick <address@hidden> wrote:

Hello,

I think its possible some of the underrun issues I mentioned in my other posts (where random gaps occur inside a block of data sent to the USRP when set the USRP sampling frequency any lower than 1 MHz,) may be solved by tweaking nout_items and buffer related settings.

For GNURadio 3.6.2, Is there a simple way to modify the python code generated by GRC in order to change those settings?  I had seen instructions for doing this with 3.6.3 but I think they don't apply to the 3.6.2 version.  I can consider upgrading to 3.6.3 if there isn't an easy solution.

Thank you, Tom

You can tweak the max noutput_items in 3.6.2. You can pass a value to the top block object when you run start/run that will set the maximum value noutput_items can be.

In 3.6.3, you now have the ability to change the maximum noutput_items on a per block basis with block.set_max_noutput_items (this might be doable in 3.6.2; I forget when we added it). You can also change the buffer sizes of a block before you start the flowgraph with set_max/min_output_buffer.

There should be very little concern going between 3.6.2 and 3.6.3. We maintain API compatibility between those versions.

But, having said all of this, I'm not sure this is really going to help you. Are you thinking of decreasing the buffer sizes or the max noutput_items? That's actually going to hurt you computationally. It will lower latency, but it means less time inside a work function and more overhead in the scheduler, moving things around, and context switching. Larger buffers mean more time in work for each block, which is when you get the most power.

Tom

 

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