Re: [Discuss-gnuradio] Signal processing and GPU

[Martin DvH]

On Mon, 2008-10-20 at 15:03 -0700, address@hidden wrote:
> Has anyone tried using GPU for signal processing?
> Does anyone know of a reason why this would not be a good idea
> I am planning on starting on it this week, would appreciate inputs

I have been working on this for quite some time now.
I did a glsl implementation a few years back but it didn't perform that
well and had some severe limitations.

So I started over this year and have reimplemented  major part of
GnuRadio using CUDA.
It is a one to one implementation.
(every gr_something block is replaced with a cuda_something block)

My work-in-progress code is at:
http://gnuradio.org/trac/browser/gnuradio/branches/developers/nldudok1/gpgpu-wip

The majority of the code is a unmodified gnuradio checkout of a few
moths back.

There are some important changes in gnuradio_core/src/lib/runtime
to support CUDA device memory as an emulated circular buffer.

I also implemented a gr.check_compare block which expects two input
streams and checks if they are outputting the same data.
I use this to check if my cuda blocks do exactly the same as the gr
blocks.

All the rest of the CUDA code is in gr_cuda.
gr_cuda has to be configured and build seperately.
Here are the cuda reimplementations of some gnuradio blocks.

Then there are new blocks cuda_to_host and host_to_cuda which copy
memory from and to the GPU device memory.

All python scripts to test and use the code are in /testbed.

The code in testbed is changing on a day-by-day basis.


There are several issues to be well aware of when doing SDR on a GPU.

-overhead
        -call overhead
        -copying data from and to the GPU
        You need to do a lot of work on the GPU in one call to have any
benefit.
-circular buffers
        -GPU memory cant't be mmapped into a circular buffer
                -solution 1: use copying to emulate a circular buffer
                -solution 2: keep track of all the processing and make your own
intelligent scheduler which does not need a circular buffer.

-threads: with CUDA you can't access GPU device memory from different
host-threads. So make sure you create use and destroy all device memory
from the same thread. (The standard GnuRadio scheduler does not do it
like this)
 
-debugging: Debugging is hard and works quite different from normal
debugging.

-parallel: The GPU is good in doing calculations in parallel which are
not dependant on each other. For this reason a FIR will perform well,
while an IIR will perform bad. An IIR can only use one processing block
of the GPU, in stead of 128.
It can still be benificial to do the IIR on the GPU when all your other
blocks are running on the GPU because you don't have to copy all samples
to the CPU, do the IIR on the CPU and copy everything back to the GPU.

All that said. I do have a complete WFM receiver which is running
completely on the GPU.
(using FIR and/or FFT filters, quadrature_demod, fm-deemph)

At the moment it is not running faster then on the CPU, mainly because
of the call overhead. (too little work items done per call)
And the extra copying done to emulate circular buffers.

I can increase the amount of work done per call by using
output_multiple. But with the current scheduling code the flow-graph can
hang. This needs work.
So the performance will change in the future.
First I want to make sure everything is working as expected.

If I benchmark a single block with a big output_multiple then I do see
performance increases.


Greetings,
Martin




> Thanks
> Inderaj
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