How do I use SAVE with a PARFOR loop using Parallel Computing Toolbox?

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I would like to save some variables to MAT files from inside a PARFOR loop. However I get an error:
??? Error using ==> parallel_function at 598
Error in ==> parallel_function>make_general_channel/channel_general at 894
Transparency violation error.
See Parallel Computing Toolbox documentation about Transparency.

Accepted Answer

MathWorks Support Team
MathWorks Support Team on 24 Feb 2020
Edited: MathWorks Support Team on 24 Feb 2020
Transparency is violated by the SAVE command because in general MATLAB cannot determine which variables from the workspace will be saved to a file.
The solution is to move the call to SAVE to a separate function and to call that function from inside the PARFOR loop. Pass any variables that are to be saved as arguments to the function. That way MATLAB can determine which ones will be saved and transparency is not violated.
For example:
Save the following as "parsave.m":
function parsave(fname, x,y)
save(fname, 'x', 'y')
Then run it with:
parfor ii = 1:4
x = rand(10,10);
y = ones(1,3);
parsave(sprintf('output%d.mat', ii), x, y);
Similarly, for loading several files with enumerated file names, we can use the following code.
function [output] = par_load(i)
eval(['load file_' num2str(i) '.mat']);
output = time;
Which can be run with:
parfor i=1:4
data = par_load(i);
a(i) = 10 + data(1);

More Answers (1)

Daniel on 14 Mar 2016
A nice alternative to implement your own save function is using the built-in matfile function:
parfor ii = 1:4
m=matfile(sprintf('output%d.mat', ii),'writable',true)
x = rand(10,10);
y = ones(1,3);
Here the parser has no issue understanding which variables are used.


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