How to use properly parfor?

Hello, I'm try to use "parfor" in order to execute 2 parallel actions: write frames to buffer & read frames from buffer.

My code is as follows:

parpool('local',4);
parfor i=1:2
  [s,f] = bufferFUNC(i,vid,fps);
end

vid is my videoinput object. fps is the predefined FrameRate.

my bufferFUNC is as follows:

function [s,f] = bufferFUNC(i,vid,fps)

      global bufferFrames

      if i == 1 % read data from buffer
          [pos,~,~,~] = Frames2Sound(bufferFrames,fps,'FALSE','FALSE','TRUE','FALSE');
          [s , f , ~] = spectrogram(pos(1,:),kaiser(64,7), 62 , 64 , fps);
      end

      if i == 2 % write data into buffer
         start(vid);
         wait(vid);
         frames = peekdata(vid, get(vid,'FramesAvailable'));
         bufferFrames = squeeze(frames); 
      end  

end

For some reason, when running in parfor configuration, I receive the following error:

Error using bufferFUNC (line 11)
An UndefinedFunction error was thrown on the workers for 'start'.  This might be because the file containing 'start' is not
accessible on the workers.  Use addAttachedFiles(pool, files) to specify the required files to be attached.  See the
documentation for 'parallel.Pool/addAttachedFiles' for more details.

Caused by:
    Undefined function 'start' for input arguments of type 'struct'.

Any idea why this is happening? If I ran the same script but with regular for then no problem... Why the fact that I'm using parfor causing MATLAB not to recognize the function "start"???

THANKS

Answer 1

[Edric Ellis]

I think you need to create vid on the workers, I suspect the object does not correctly transfer from the client to the workers because they are separate MATLAB processes. You could try doing something like:

    parfor idx = 1:4
        vid = videoinput(...);
        % use 'vid'
        delete(vid);
    end

If that works, you can use parallel.pool.Constant (or WorkerObjWrapper if you're not using R2015b), something like this:

    vidWrapper = parallel.pool.Constant(@() videoinput(...), @delete);
    parfor idx = 1:2
        vid = vidWrapper.Value;
        % use 'vid'
    end
    % Note that when 'vidWrapper' goes out of scope, 'delete' will be invoked
    % on the object that was created.

[Mark Golberg]

Thank you Edric. That indeed solves the problem of object (vid) assignment withing the workers.

But, now I've encountered a new (more fundamental) problem:

I want to simultaneously read and write from my buffer.

My idea was to do something like:

parfor i=1:2
    if i==1
      ** read from buffer **
    end

    if i==2
       ** write to buffer **
    end
end

BUT, I receive the following error:

An UndefinedFunction error was thrown on the workers for 'bufferFrames'.  This might be because the file containing
'bufferFrames' is not accessible on the workers.  Use addAttachedFiles(pool, files) to specify the required files to be attached.
See the documentation for 'parallel.Pool/addAttachedFiles' for more details.

Caused by:
    Undefined function or variable 'bufferFrames'.

If I am not mistaken its because my bufferFrames variable is common to both i=1 & i=2. BUT that's exactly what I would like to do - read frames from the buffer and then write NEW frames (that just came in from the live camera) into the same buffer.

Is there a workaround?

[Walter Roberson]

global are not transferred between workers

[Edric Ellis]

Ah, thanks @Walter, I hadn't spotted the use of global in there too - yes, global data is not synchronised.

If you need communication between the workers, you need to look into spmd blocks rather than parfor, and use labSend and labReceive to transfer data.