Cancel parfeval with a uiprogressdlg

Hello,

I'm making a UI that works in parallel and a uiprogressdlg is running while it's not finished.

I'm trying to make the cancel button of this uiprogressdlg do something but it's like nothing is happening when I push it.

I tried to put a line in me second loop above. It does nothing since when I push the button the code is doing the fetchNext line and when the fetchNext is done, every job are done...

I also add a listener to the button, but nothing happens when I push the cancel button...

Any hint how to stop all working jobs by hitting this cancel button ?

function testCancel()
    clearvars
    close all
    clc
    
    figT = uifigure('MenuBar','none','toolbar','none','NumberTitle','off','Name','main window');
    uibutton(figT, 'push', 'Text', 'run', 'ButtonPushedFcn',{@run_Callback});
    
    function run_Callback(~,~)
        tic;
    
        progressDlg = uiprogressdlg(figT, 'Title','running', 'Indeterminate','on', 'Cancelable', 'on');

        pool = gcp("nocreate");
        if isempty(pool)
            parpool('local');
            pool = gcp("nocreate");
        end
        
        future = parallel.FevalFuture();
        for i=1:5
            future(i) = parfeval(pool, @run,1, 5e7);
        end
        addlistener(progressDlg, 'CancelRequested', 'PostSet', @(src,event)disp('run canceled'));
        for i=1:5
            % if progressDlg.CancelRequested
            %    cancelRun(future);
            % end
            [~,tot] = fetchNext(future);
            disp(tot);
        end
                            
        delete(progressDlg);
    
        toc;
    end

    function tot = run(nb)
        tot=0;
        for i=1:nb
            tot = tot + rand(1);
        end
        tot = tot/nb;
    end

    function cancelRun(future)
        disp('run canceled');
        cancel(future);
    end

end

Answer 1

[Raymond Norris]

Hi @Sylvain Chupin. A couple of comments/suggestions

When you start up the pool,

        pool = gcp("nocreate");
        if isempty(pool)
            parpool('local');
            pool = gcp("nocreate");
        end        

the parpool command will return a handle to the pool object, so there's no need to call gcp again.

        pool = gcp("nocreate");
        if isempty(pool)
            pool = parpool("local");
        end  

Then again, since you want a pool to run (and start if it hasn't), I would replace all of this with just

        pool = gcp;

This slight difference here is, I'm saying "give me a handle to a pool, if one doesn't exist, use the default profile." Where yours says, "...if one doesn't exists, use the local profile."

Note also that your tic/toc includes the pool startup (if it hasn't already started).

As you saw, you can't add a listener to the uiprogressdlg. I suspect that is why the CancelRequested property exists. It's perhaps the listener you should be using (and have commented out).

I would replace the for-loop with a while loop, as such (and it has the benefit that you don't care how many futures you add)

        while true
            if progressDlg.CancelRequested
                cancelRun(future)
                break
            else
                if any(~contains({future.State},'finished'))
                    [~,tot] = fetchNext(future);
                    disp(tot)
                else
                    % No more future to fetch
                    break
                end
            end
        end        

If this doesn't work, I'll post the entire code, but I believe that was the only change I made. I should point out, what you can't do (and what you might be trying to do) is cancel the future you're currently listening to. That is, once you've checked that you haven't canceled the progress bar, MATLAB is blocking until it's finished fetching the next future. Before it checks for anymore futures, it'll check for the cancel again.

For this to be a bit more obvious, add

        pause(rand*15)

In the run function before the for-loop. This will skew the runtimes a bit more for each of the parfevals (I'm assuming this is just an example you're showing us).

[Sylvain Chupin]

Thanks a lot for your answer and your comments.

It work pretty well.

I just changed one thing : in your version, the cancel function can only be called after the first fetchNext. I change it a bit to call the cancel function as soon as we push the button.

        nbPar = 5;
        for i=1:nbPar
            future(i) = parfeval(pool, @run,1, 5e7);
        end
        
        fetched = 0;
        while true
            if progressDlg.CancelRequested
                cancelRun(future)
                break
            else
                if fetched == nbPar
                    break
                else
                    if any(contains({future.State},'finished'))
                        [~,tot] = fetchNext(future);
                        fetched = fetched + 1;
                        disp(tot)
                    end                    
                end

            end
        end        

Thanks again

[Raymond Norris]

Good catch @Sylvain Chupin. You can simplify this a bit

        while true
            if progressDlg.CancelRequested
                cancelRun(future)
                break
            elseif fetched == nbPar
                break
            elseif any(contains({future.State},'finished'))
                [~,tot] = fetchNext(future);
                fetched = fetched + 1;
                disp(tot)
            end
        end        

And since canceling finished tasks is a no-op,

        while true
            if progressDlg.CancelRequested || (fetched == nbPar)
                cancelRun(future)
                break
            elseif any(contains({future.State},'finished'))
                [~,tot] = fetchNext(future);
                fetched = fetched + 1;
                disp(tot)
            end
        end        

Though this might be a bit more odd to read (since why would you cancel finished tasks?).

[Sylvain Chupin]

Indeed it's even simplier like this.

For canceling the finished tasks, in my real case, I changed the cancelRun function to cancel only the unfinished tasks.