Help with parfor progress bar using data queue

Question

HpW 2020-11-24

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/660793-help-with-parfor-progress-bar-using-data-queue

评论： HpW 2020-11-30

采纳的回答： Edric Ellis

在 MATLAB Online 中打开

Hello. I am trying to implement a progress par for a parfor loop, following the example shown at

https://www.mathworks.com/help/parallel-computing/parallel.pool.dataqueue.html#bvo5uvj-1

I am using a script instead of a function as shown in the example, but I am unable to adapt the code in a way that works.

My code:

clear
D = parallel.pool.DataQueue;
h = waitbar(0, 'Please wait ...');
afterEach(D, @nUpdateWaitbar);
num_files = 1000;
parfor i = 1:num_files
    % do stuff
    send(D, [i num_files]);
end
function p = nUpdateWaitbar(input)
    p = input(1)/input(2)
    waitbar(p, h);
end

Executing this gives

Warning: Unrecognized function or variable 'h'.

which I assume is because the function nUpdateWaitbar doesnt know what 'h' is

but if I try to pass the waitbar handle 'h' into the dataqueue, I get another error:

Cannot convert double value 1000 to a handle

Can you please point me towards what I am doing wrong?

the parfor loop runs fine without the waitbar code so I dont think that is the issue

thanks!

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Mario Malic 2020-11-24

编辑：Mario Malic 2020-11-25

在 MATLAB Online 中打开

Try this

function p = nUpdateWaitbar(input)
    persistent h
    if isempty(h)
        h = findall(0, 'type', 'figure', 'Tag', 'TMWWaitbar'); 
    end
    p = input(1)/input(2)
    waitbar(p, h);
end

HpW 2020-11-25

编辑：HpW 2020-11-25

在 MATLAB Online 中打开

thanks!

I was able to make it work this way:

function nUpdateWaitbar(input)
        h = findall(0, 'type', 'figure', 'Tag', 'TMWWaitbar'); 
        p = input(1)/input(2)
       
        waitbar(p, h);
        
    end

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Answer 1

Edric Ellis 2020-11-25

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/660793-help-with-parfor-progress-bar-using-data-queue#answer_555583

在 MATLAB Online 中打开

The problem here is that the documentation example is using a nested function, which is able to acces variables in the containing workspace. Your function definition inside a script is a local function, which cannot automatically access the variables in the containing workspace. An alternative fix is to use an anonymous function handle to "bind" in the value of h.

clear
D = parallel.pool.DataQueue;
h = waitbar(0, 'Please wait ...');
% Note anonymous function captures the value of "h" to pass
% in to "nUpdateWaitbar"
afterEach(D, @(data) nUpdateWaitbar(data, h));
num_files = 1000;
parfor i = 1:num_files
    % do stuff
    send(D, [i num_files]);
end
function p = nUpdateWaitbar(input, h)
    p = input(1)/input(2)
    waitbar(p, h);
end

This "works", but it isn't right. If you run it you'll see the value flickering about because the parfor loop iterations are not processed in order. So, you need a different approach for nUpdateWaitbar. I would essentially make nUpdateWaitbar contain persistent data and have an "initialisation" mode. Like this:

clear
D = parallel.pool.DataQueue;
h = waitbar(0, 'Please wait ...');
num_files = 1000;
% Dummy call to nUpdateWaitbar to initialise
nUpdateWaitbar(num_files, h);
% Go back to simply calling nUpdateWaitbar with the data
afterEach(D, @nUpdateWaitbar);
parfor i = 1:num_files
    % do stuff
    % Note we send only an "increment" for the waitbar.
    send(D, 1);
end
function p = nUpdateWaitbar(data, h)
persistent TOTAL COUNT H
if nargin == 2
    % initialisation mode
    H = h;
    TOTAL = data;
    COUNT = 0;
else
    % afterEach call, increment COUNT
    COUNT = 1 + COUNT;
    p = COUNT / TOTAL;
    waitbar(p, H);
end
end

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Edric Ellis 2020-11-30

persistent is more tightly-scoped than global - so you don't have to worry about whether other bits of code might interfere. (In general, it's good practice to try and keep your data as tightly-scoped as possible). In my 2nd example above, nUpdateWaitbar is a "local" function inside the script - this means it is only accessible to code within that script file.

HpW 2020-11-30