Re: Reducing rare underflows to zero occurrences

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Hi Chad,

Something that you can do to monitor the problem is to look in
/proc/net/udp. This will show you the fill level of kernel UDP receive
buffers. If you've set the buffer size to 32 MiB as recommended by the
performance tuning tips, usually these sporadic drops are the result of
the flowgraph consuming data at a rate slightly lower than the nominal
sample rate for multiple seconds, rather than a brief but larger
performance reduction. If this is the case, you can see how the buffer
fill level slowly increases over the course of several seconds until the
buffer fills up and packets are dropped. Looking at this won't solve
your problem, but at least it will give you a way to monitor if you do
some changes to try to improve the performance.

Best,
Daniel.

On 16/01/2025 19:38, Chad Spooner wrote:
> All:
>
> I'm developing a comm system involving three SDRs: transmit, channel
> effects, and receiver.
>
> The channel SDR is controlled by a flowgraph that has various options
> for the channel: AWGN (h(\tau) = \delta(\tau)), LTI, and a couple time-
> variant frequency-selective channel models.
>
> I need to run the full three-SDR system over long periods of time to get
> stable average values for system-performance parameters.
>
> My problem is that the channel flowgraph reports underflows on the order
> of every hundred seconds or so (sometimes much less). I've tracked down
> these rare underflows in the channel to transient problems in the
> receiver flowgraph.
>
> So I put together a minimum flowgraph that shows the problem, which is
> attached. When I run it, I get output like this:
>
> [INFO] [UHD] linux; GNU C++ version 11.2.0; Boost_107400; UHD_4.1.0.5-3
> [INFO] [X300] X300 initialization sequence...
> [INFO] [X300] Maximum frame size: 8000 bytes.
> [INFO] [X300] Radio 1x clock: 200 MHz
> usrp_source :info: set_min_output_buffer on block 1 to 2097152
> usrp_sink :info: set_min_output_buffer on block 2 to 2097152
> mmse_resampler_cc :info: set_min_output_buffer on block 4 to 2097152
> fir_filter_blk<IN_T,OUT_T,TAP_T> :info: set_min_output_buffer on
> block 5 to 2097152
> multiply_cc :info: set_min_output_buffer on block 9 to 2097152
> sig_source :info: set_min_output_buffer on block 8 to 2097152
> add_cc :info: set_min_output_buffer on block 6 to 2097152
> fastnoise_source :info: set_min_output_buffer on block 7 to 2097152
> *Uusrp_sink :error: In the last 579300 ms, 1 underflows occurred.
> Uusrp_sink :error: In the last 256008 ms, 1 underflows occurred.
> usrp_sink :error: In the last 84992 ms, 1 underflows occurred.
> UUusrp_sink :error: In the last 128008 ms, 1 underflows occurred.*
>
> The PSD of the filtered input signal appears correctly on a spectrum
> analyzer.
>
> System Information:
>
> 24-core Intel i9 system
> Ubuntu 22.04.5 LTS, kernel 5.19.0-46-generic #47~22.04.1-Ubuntu
> gnuradio 3.10.1.1
> UHD 4.1.0.5-3
>
> I've taken some pains to optimize the OS settings for gnuradio/UHD along
> the lines suggested by the online tips at
>
> https://kb.ettus.com/USRP_Host_Performance_Tuning_Tips_and_Tricks
>
> I've also tinkered with the minoutbuf parameter in the various blocks.
>
> Any advice?
>
> Thanks,
>
> Chad
> cyclostationary.blog
> cmspooner@nwra.com
>
>
>
>
>
> --
> Chad M. Spooner, PhD
> NorthWest Research Associates
> 149 Bonifacio Place, Suite 200
> Monterey, CA 93940
> cmspooner@nwra.com
> 831 521 6743
> NWRA Scientist/Engineer, VP, BoD
> cyclostationary.blog
>