[USRP-users] polyphase clock sync eats 100% cpu once it gets samples

Marcus Müller marcus.mueller at ettus.com
Tue Oct 3 07:57:07 EDT 2017

Hi Vladimir,

synchronization is usually among the most CPU-intense things a receiver
does (only, if at all, contested by channel decoding for complex codes).
So, the 100% CPU utilization don't sound totally unreasonable, depending
on your system.

That being said, I don't want to rule out bugs, but for the time being,
I'd declare this issue as "unclear, probably insufficient compute power".

Can you tell us a bit about your computer, in terms of CPU model,
motherboard chipset, RAM configuration, OS? If you install and run
"htop"¹, you'll see which block does how much without much complication,
and maybe also significant non-GNU Radio CPU usage (for example, my mail
client and my browser idling use *serious* amounts of CPU).

Another thing worth trying is to close all software that might be using
CPU (check with htop!) and then run "volk_profile"; this should test a
lot of hand-written implementations for certain math operations, which
might significantly speed up the polyphase clock sync.

Best regards,


¹: I'm assuming you're using windows; after starting htop, press F2 for
setup, go into the "Display Options", enable "Show custom thread names",
press Esc

On 03.10.2017 12:54, Vladimir Rytikov via USRP-users wrote:
> Hi,
> I am trying to run an example from GNU Radio -
> examples/digital/packet/uhd_packet_rx and uhd_packet_tx with real USPR
> radios connected via an attenuator and a coax cable.
> When I enable receiver by clicking on 'On' check box - the whole RX
> flow graph freezes.
> I think I manually adjusted transmit power and receive gain to be
> within reasonable ranges.
> By disabling different blocks one at a time I found is that polyphase
> clock sync inside packet_rx blocks kills the flow graph. It seems like
> the signal is gaited by Correlation Estimator and once it gets the
> correct sync word - the signal goes to Polyphase Clock Sync and CPU
> dies - it is 100% loaded all the time.
> If I run only loop back simulation the whole flow graph seems working
> fine. I wonder if noise or not very good signal destroys Polyphase
> Clock Sync.
> I installed GNU Radio via PyBOMS - 3.7.12git* version. Please help to
> find better idea on debugging it.
> with a different simple flow graphs Polyphase Clock Sync works fine
> even when I feed noise to it and signal later.
> Thanks,
> Vladimir 
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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