Re: [Discuss-gnuradio] Try to improve E100's performance at high sample rate

[ziyang]

On 01/17/2012 07:36 PM, Josh Blum wrote:
> On 01/16/2012 09:51 AM, ziyang wrote:
> > On 01/13/2012 09:30 PM, Josh Blum wrote:
> > > > To reduce the computation load of the processor, I tried two methods:
> > > > 1) modify the gr.quadrature_demod_cf block, replace some multiplication
> > > > operations with volk-based operations (gr.multiply and gr.multiply_const
> > > > modules in gr_blocks);
> > > I like it. Make sure to contribute patches like that back. :-)
> > Actually, what I did was writing a new quadrature_demod block without
> > the multiplication and delay operations, and connect extra gr.multiply
> > and gr.delay blocks instead in the flow graph. Because my understanding
> > is that the volk functions take a vector (multiple values) as input, and
> > I didn't figure out a way to do the single-item-operation in the volk
> > style.
> I dont recommend using the extra blocks, that would probably cause more
> overhead. Looking at gr_quadrature_demod_cf::work, it looks like you can
> vectorize the operation of the conjugate multiply, then the atan, then
> the gain scaler. So, that would be one for loop that operates on 4
> samples at a time, and calls 3 volk functions.

Josh, thank you for your advice! Before I tried using gr.multiply out of 
the block, I actually implemented a demodulation block in a way that's 
similar to your suggestion, but the loop operated on 100 samples at a 
time. I don't know if it was the 100-samples-vectorization that caused a 
bad performance. I will try processing 4 samples at a time.

> > > Also, you may consider timing a particular operation as a performance
> > > metric, rather than counting the number of demodulated packets.
> > I was wondering if there are examples from which I can learn how to do
> > this?
> Sorry, I guess there isnt much in the way of examples.
>
> You can time individual work functions by adding some code before an
> after. We have some high resolution timers in
> gruel/include/gruel/high_res_timers.h

So I call the timer functions of high_res_timers.h before and after the 
operation in the work function, is that right?

> I have also seen people time the block in a simple flow graph with a
> null source, head, your_block, null_sink. You can time tb.run() and
> compare run duration vs the non-vectorized code.
>
> -Josh

I got two questions about this:

1) Is the "head" block for generating data for the processing block?

2) The initialization of uhd is done first after tb.run(), so how could 
I isolate the processing time from the time between tb.run() - tb.stop() ?

Thanks.


Best Regards,

Terry

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