[USRP-users] performance issues

Josh Blum josh at joshknows.com
Mon Nov 29 01:42:57 EST 2010

On 11/28/2010 03:39 PM, Juha Vierinen wrote:
> Hi,
> I recently upgraded to the newest protocol 7 of uhd. I noticed that I
> am having trouble getting all the packets in.

Nothing drastic in the receive path (to the host) has changed for quite
some time. Especially if transmit is not involved. So I am befuddled.

> When I run benchmark_rx_rate, I notice that even with 4 MHz, the
> process is taking over 50% of the CPU time, and at 25 MHz it is taking
> 150 % (it is threaded I guess). Why is this? The usrp1 only takes
> about 15 % of the CPU time when running at 5 MHz, even when writing
> everything to a file.  I rember the gnuradio driver used much less CPU
> resources. What is going on?

There is a thread in the receive path that inspects packets and places
them into a queue. You are most likely seeing the extra overhead (vs
gnuradio) of this thread. I am looking into removing it in the near
future if doing so decreases the CPU overhead.

Greater overhead is just a reality given the additional features w/ UHD,
but it should not be drastic. I can look into this more when I get back
into town.

> Is there e.g., any way to increase the packet size? I have a hunch
> that this might help.

You can bump up the recv_frame_size

Assuming your network card can handle frames over 1500 bytes, the USRP2
can output frames of up to 2048*4 bytes including transport headers (I


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