Parfor loop stuck when saving plots

Hello everyone,

I am encountering a strange issue when using a parfor loop to perform calculations on a set of data and saving the plots.

I used to do this for my files and to plot and save multiples data (2D data of electrical field, 1D profile, FWHM size as function of the file, etc) and I never had any issue.

However, I recently try to do the same for a 3D plot (using surface function) and then the parfor loop does not respond and stop doing anything (but still my CPU is 100 %...). It happens wether I used 1 or 16 (my max) workers, and in 2 differents PC with R2020a.

While debugging, the issue is coming from the print (or saveas, I tried both) command...but this happened only for the full size of my data (X, Y and E are 2000x2000, with each a 32 MB size) ; if I plot for example only 300x300 points it works. [Furthermore I have up to 128 GB of RAM memory and never more than 20 GB is used)

I also try to plot 600x600 with 1 worker or 300x300 with 16 workers and the first case is blocked while the second one works. Also, if I put a condition in my loop so that the saving is done only in the first case (k=1), then every workers are running except the one that met that condition. What is more, if I change X and Y to a 1D array (1x2000) and keep E 2D (2000x2000), then a imagesc plot is working without issue, so there is no issue with my data I believe.

And, of course, if I run the same code with a normal for loop, everything goes well (but very slooooow !)

I have no more ideas about that, so if you have some suggestions I will be glad :)!

Here is an example of my code:

myfolder = pwd;
destdirectory = [pwd,'/3Dsave'];
mkdir(destdirectory); 
L = dir('./*dat');
parfor k=1:length(L)
    cd(myfolder)
    pause(0.01);
    N = L(k).name;
    Md = table2array(readtable(N)); 
    x = Md(:,1);
    nx = length(unique(x));
    
    
    y = Md(:,2);
    ny = length(unique(y));
    
    yl = reshape(y,[nx,ny]);
    xl = reshape(x,[nx,ny]);
    Ez = Md(:,3);
    Ez2 = reshape(Ez,[nx,ny]);
    Ez2=abs(Ez2.^2);
    
    fl=figure('visible','off');
    surface(xl*1e6,yl*1e6,Ez3)
    shading interp
    colormap jet
    set(gca,'FontSize',11,'fontweight','bold')
    hcb=colorbar;
    hcb.Title.String = "A_0^2";
    title(N)
    cd(destdirectory)
    pause(0.01)
    print(['iml_',num2str(k),'_apng'],'-dpng','-r1200')
    close(fl);
end

[Rik]

I don't have the solution for your problem, but you shouldn't have cd that far in your code. You should use fullfile to generate the file names. You also have several calls to the length function. While they aren't causing any bugs here, you should never use it. I've never seen an situation where length did something that numel and size couldn't. (although I must admit that you can use it to test if an array is a vector by using length(x)==numel(x))

I would personally add explicit graphics handles to all calls. That is unlikely to fix the problem, but at least you remove one factor.

Did you try this code with a normal for-loop?

[Alexandre]

Thank you for the suggestions, I did not know the fullfile function. For the length function, I always prefer to use it even with non-square matrix (in that case I do something like length(A(1,:)).

For the graphics handles, I try to use something like h=surface(xl*1e6,yl*1e6,Ez3), but it did not change anything.

And yes the code has been tested with a regular for-loop and it is working with no issue.

[Walter Roberson]

length(A(1,:)) is significantly less efficient than size(A,2) . A(1,:) has to first extract all of row 1 from the matrix, even though the row elements are not going to be in consecutive memory (unless it just happened to be a vector). This results in a bunch of unnessary memory copying.

length(A(1,:)) also fails if A has no rows, such as if it A were 0 x 10 . You would get an error rather than either 0 or 10.

size() is one of the very fastest operations for any class that does not override the definition of the size() method: it only has to look at the header block for the expression. ( sym is an example of a class that redefines size. )

[Rik]

For the graphics handles I meant you should specify the parent object in every call that supports it. Perhaps that will help.