Re: [Discuss-gnuradio] noutput_items in every block

[Sakthivel Velumani]

Hi Michael,

Thank you very much for the detailed explanation. I have one more query - If a block has two input streams, will the no of items be same in both streams? say for example I build a block that takes I and Q samples as input and the algorithm demands I sample and its corresponding Q sample to work correctly. In this case does the scheduler guarantee that the items in both buffer are of same number and in the same order? or do I have to check that I am processing every Q sample and its corresponding I sample using tags or some other mechanism?

Best

Sakthivel

On Mon, Jan 1, 2018 at 5:06 PM, Michael Dickens <address@hidden> wrote:

Hi Sakthivel - Short answers: The value can vary for each call; it is determined by the scheduler. I've provided more info below if you're curious. Cheers! - MLD

Details: One way to think of your questions is to imagine the finite-length I/O buffers that hold the data between blocks, and note that, in general, it is more CPU efficient to process "more" data than very small chunks -- typically 1k of data can be handled more efficiently than 4 bytes, when you consider the CPU overhead required for the scheduler (this is true up to some "large" data amount, when processing efficiency peaks and possible even drops somewhat off of peak). When the flowgraph starts, these buffers are all empty; so the scheduler tries to get blocks to process as much input data as possible. Once the flowgraph is running, the buffers hold (for all practical purposes) random amounts of data, which means that the blocks (in general) will not be able to process the amount of data as at startup time. Data will flow roughly in bursts from source to sink, but since each block is executing in its own thread the end result is data pipelining: "work" for any specific block happens when there is simultaneously "enough" input data and "enough" output buffer space -- combine the "note" above with this concept and you have a rough interpretation of the scheduler algorithm. Thus, with data streaming the scheduler has to be able to work with dynamic amounts of I/O data / buffer space.

On Mon, Jan 1, 2018, at 10:30 AM, Sakthivel Velumani wrote:
> I am new to GNU radio. I have this general doubt that when items are streamed from one block to another, how many input_items per port (consider a type general block) are passed when the work() function of the block is called each time? I guess this is handled by the GNU radio's scheduler but would like to know if this is constant or it varies for each call?