Re: [Discuss-gnuradio] Threaded GRC blocks

[Marcus Müller]

:)

I have no doubt I'm doing GNU radio wrong,

I might have put that a little harsh; sorry. You're not doing it wrong, you just weren't aware of a few core concepts of the GNU Radio scheduler:

but I'm a bit confused as to how to implement this otherwise, as I don't want my processing completed on every block coming in and I don't want samples to stop while I'm processing

And exactly that's the kind of worries GNU Radio strives to take away from you.
All blocks in your flow graph can run in parallel. So while your block is still working, the upstream block is already processing what is going to be the input of your block's next iteration, while your downstream block is busy processing what your block produced the last time.
Also, GNU Radio uses input and output buffers, which are identical to your upstream block's output and your downstream blocks' input buffers, respectively, and makes sure you know how many space there is in these buffers (or how much samples there are for you to process).

This all is done transparently in separate threads, so you don't have to worry about it.

I hope that illustrates why I think that spawning your own thread is not necessary; it's already being done for you, and in a manner that allows you not to care about the correct transportation of data, notifying threads, ensuring data flow and proper multiprocessor scaling -- this all happens behind the curtains. To the user, each block only has to care about processing its input as fast as possible to produce output; the scheduler will coordinate everything else.

In fact, GNU Radio even encourages you to think about how you can further break down your algorithm, to as well avoid re-inventing the wheel, and to use optimized algorithms.
Maybe you do an FFT inside? Well, then use the existing GNU Radio FFT. Are you multiplying to sample streams? Do that with a GNU radio multiply block, and you'll profit from SIMD-optimized routines.
Also, when breaking down an algorithm into existing and to-be-written blocks, you increase the level of parallelity, which generally is a good thing, because buffering necessary to avoid congestion when a step takes especially long will then be automatically distributed between a lot of blocks.

Greetings,
Marcus

On 01/16/2015 06:45 PM, Jon West wrote:

Thanks for the reply. 

I'm doing a ranging application, and the wake up times are related to the signal period of my ranging signal. The application can take up to 500ms to search for my reference signal in certain conditions and that can be improved with some prediction, but the first go round can take a while. I have no doubt I'm doing GNU radio wrong, but I'm a bit confused as to how to implement this otherwise, as I don't want my processing completed on every block coming in and I don't want samples to stop while I'm processing

On Fri, Jan 16, 2015 at 12:24 PM, Marcus Müller <address@hidden> wrote:

Hi Jon,

I'll try to structure this and reply in-text, so we can get to a mutual understanding faster :)

> My application buffers a bunch of data and then performs some signal processing on it that can take up to 500ms.

Does that mean it takes up to 0.5s worth of sampled signal, or does just the computation take that long? Is there something like a minimum block size of samples that your algorithm needs?
Here, a bit of info on what you're actually doing would be nice.

> Once processing is complete, the processing thread waits a certain amount of time before reading the buffer and then processing again, meanwhile the main trhread is consuming samples and advancing a sample counter.

GNU Radio will do exactly that for you: you just write a block that transforms a set of input items to a set of output items, and GNU Radio cares about how to fill your input buffer, when to call you, how to inform you how much items there are to process, and how to notify your downstream flowgraph neighbors about new data.

> I was wondering what the best way to implement this as a GRC block.

Depends on what you do in that block. I have my doubts about your 500ms computation step not being split into smaller processing steps; but the feasibility of that completely depends on the actual thing you want to do...

> Currently I am creating the thread in the the block constructor and killing it in the destructor.

That sounds a bit like you're doing GNU Radio wrong. Your block is already running in a thread of its own -- that's what the thread-per-block scheduler does for you ;)

Greetings,
Marcus

On 01/16/2015 06:13 PM, Jon West wrote:

I'm new to gnu radio, but I am trying to port a thread SDR application in to  a GRC block. My application buffers a bunch of data and then performs some signal processing on it that can take up to 500ms. Once processing is complete, the processing thread waits a certain amount of time before reading the buffer and then processing again, meanwhile the main trhread is consuming samples and advancing a sample counter. I was wondering what the best way to implement this as a GRC block. Currently I am creating the thread in the the block constructor and killing it in the destructor. I've done a search to try and find a solution to this but not finding much, or constantly be directed at the same results that don't help

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