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Re: [Discuss-gnuradio] usrp2::tx_16sc calling rate

From: Gaetano Mendola
Subject: Re: [Discuss-gnuradio] usrp2::tx_16sc calling rate
Date: Wed, 16 Dec 2009 02:29:54 +0100

On Tue, Dec 15, 2009 at 8:51 PM, Eric Blossom <address@hidden> wrote:
> On Tue, Dec 15, 2009 at 01:01:22AM +0100, Gaetano Mendola wrote:
>> Hi all,
>> I think I'm experiencing some buffer underrun using an instance of
>> usrp2, to transmit my
>> samples I'm using the method usrp2::tx_16sc.
>> I have the usrp2 equipped with a rfx400 and using as interpolation a value 
>> of 8.
>> I found out that 192 calls to usrp2::tx_16sc last 0.56 seconds passing
>> to it a total of 7104000
>> complex. At my rate 7104000 complex are transmitted in air in a bit
>> less than 0.56 seconds
>> and it seems I'm very tight in the time I can loose between two
>> usrp2::tx_16sc calls.
> At interp = 8, you need to generate 12.5MS/s, so your average rate
> looks OK (710400/0.56 = 12.67MS/s), but you'll want to check this
> overa longer window, say 10s.

The data I was reporting was an average on 192 calls over more than
20 seconds of transmission.

> Have you enabled real time scheduling in your transmitter code?
>  #include <gruel/realtime.h>
>  gruel::rt_status_t r = gruel::enable_realtime_scheduling();
>  if (r != RT_OK)
>    std::cerr << "Failed to enable realtime scheduling\n";
> For this to work, you'll need to be in group "usrp" and will have to add
> this line to /etc/security/limits.conf:
> address@hidden   -  rtprio 50

That was the next inline activity I had plan to do.

>> My questions are:
>> .) How do I check if I had a real underrun?
> Right now you can't unless you hook up a cmos-to-serial port converter
> to the USRP2.  (This is being fixed as part of the VRT work).

No idea what VRT is, I will google for it :D

>> .) How much time I can loose roughly between two calls to not incur in
>> underruns?
> There's a small amount of buffering on the USRP2. It's on the order of
> a few ethernet frames.  The host needs to pretty much not miss a
> beat to keep it fed.

At my rate 2 ethernet frames are realy not that much :s

>> .) Is there a way to make the tx_16sc async ?
> Sure, put the caller in it's own thread.  Or use GNU Radio to talk to
> the USRP2.  We already do that.

The caller is already on his own thread and between the samples producer
and the thread calling the tx_16sc I have a double buffered queue that permits
small interactions between consumer and producer apart a small window time
in where the two buffers are swapped. I have to replace those two queues with
a rings in order to eliminate the need to do heap allocations.

actualy my caller thread main loop is something like this:

1) samples* p = std::list<samples*>::front;
2) 192 calls of usrp2::tx_16sc(...);
3) delete p;
4) std::list<samples*>::pop_front;

I see that the average time between two calls of those192 tx_16sc has some
spikes, and those spikes are related to the average time for that
delete. I hope
that changing the scheduler, and removing the dynamic memory allocation my
problem is solved. I'll let you know otherwise :D

Thank you for your explanations.

Gaetano Mendola


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