Communicate with worker through client

Hello!

I have an app (app designer/GUIDE) which calls a function that looks something like this

function func()
    spmd
        switch labindex
            case 1
                while (true)
                    % some code...
                end
            case 2
                while (true)
                    % some code...
                end
        end
    end
end

and I would like to be able to break out of the while loop for each workers of the spmd with a click of a button in the app, which means I have to send data from the client to the workers.

I know I can easily send data from the workers to the client, but I'm not sure about the reverse direction.

Please help, thanks!

Answer 1

[Edric Ellis]

I think it would be better to use parfeval or parfevalOnAll to do this. This way, you can simply cancel the future object returned from those calls to stop the worker execution. You could couple this with a parallel.pool.DataQueue to get the results back from the workers. Here's a simple example:

% Build a really simple "UI" - just a histogram and a button
pool = gcp();
fig  = uifigure();
grid = uigridlayout(fig, [1, 2]);
ax   = axes(uipanel(grid));
btn  = uibutton(grid);
hist = histogram(ax, 'BinEdges', 0.5 + (0:pool.NumWorkers));

% Set up a DataQueue to let the workers send results back to the client
Q = parallel.pool.DataQueue();
% Each time a worker sends a piece of data to the client, append it
% to the histogram
listener = afterEach(Q, @(data) appendData(hist, data));

% Trigger the work on the workers, hook up the future to the button
% so that pressing the button cancels 'fut'.
fut = parfevalOnAll(@doWork, 0, Q);
btn.ButtonPushedFcn = @(~,~) cancel(fut);

%% function to do work on the workers
function doWork(Q)
task = getCurrentTask;
myId = task.ID;
while true
    send(Q, myId);
    pause(myId/4);
end
end

%% function to append data to the histogram
function appendData(hist, data)
hist.Data(end+1) = data;
end

[guy sharon]

Hi, Thank you for the answer.

However, I want to be able to have communication between the workers with labSend and labRecieve.

[Edric Ellis]

labSend and labReceive can only be used inside spmd. Unfortunately, spmd is a synchronous construct on the client, so there's no way for the client to interrupt the workers.

[guy sharon]

So there is no way to both be able to communicate between workers and be able to stop all workers from the client?

Answer 2

[Thomas Falch]

From R2025a you can use the new "any-destination" PollableDataQueue to more easily communicate between the client and workers, or between different workers. (The existing PollableDataQueue can also do this, but this requires more elaborate setup, since you always need to create the queue on it's destination).

With the any-destination queue, you can implement a stop button for an spmd block like this:

queue = parallel.pool.PollableDataQueue(Destination="any")";

spmd
    while true
        doSomeWork()
        
        % Check if we've got a stop message on the queue
        [data, didReceive] = queue.poll()
        if didReceive && strcmp(data, "stop")
            break
        end
    end
end

% In GUI stop button callback
queue.send("stop")