Using 'load' under parfor loop

Hi there,

I am trying to load a variable (E) under my parfor loop. But it is giving me an error

An UndefinedFunction error was thrown on the workers for 'E'.  This might be because the file containing 'E' is not
accessible on the workers.  Use addAttachedFiles(pool, files) to specify the required files to be attached.  See the
documentation for 'parallel.Pool/addAttachedFiles' for more details.

Caused by:
    Undefined function or variable 'E'.

I load using a seperate function as directed in http://ch.mathworks.com/matlabcentral/answers/135285-how-do-i-use-save-with-a-parfor-loop-using-parallel-computing-toolbox for saving. Can I please know if it is not possible to load under parfor?

Thanks.

Answer 1

[Walter Roberson]

It is possible to load(), but you have to addAttachedFiles() the file to the pool.

The workers operate in something that functions much like a virtual environment, which only have access to the routines that are directly called (static analysis of calls), and to the routines and data files which have been specifically passed in by attaching them.

[Rachel Chen]

When do I add addAttachedFiles() ? I tried:

1. Adding it just after starting parfor

load('my_variables.mat');                                   % variables a and b
myMatrix = zeros(length(a), length(b));
for i = 1:length(a)
    parfor j = 1:length(b)
        poolobj = gcp;
        addAttachedFiles(poolobj, {'my_variables.mat'});    
        %load('my_variables.mat')
        hi = a(i)*10 + b(j)*10^2;
        myMatrix(i,j) = hi;
    end
end

2. Adding it before starting parfor

load('my_variables.mat');                                   % variables a and b
myMatrix = zeros(length(a), length(b));
for i = 1:length(a)
    poolobj = gcp;
    addAttachedFiles(poolobj, {'my_variables.mat'});        
    parfor j = 1:length(b)
        hi = a(i)*10 + b(j)*10^2;
        myMatrix(i,j) = hi;
    end
end

But both didn't work.

Method 1 throws out the error using

parallel.Pool.addAttachedFiles (line 23) - Expecting input number 1, pool, to be non-empty. 

Method 2 displayed "

Analysing and transferring files to the workers....done

. BUT it threw out the exact same issue OP brought out.

What should I do? If it helps, the parfor loop is inside a function which is called by another MATLAB code

[Makiko Suitani]

I think you should call the external function and load it in the external function.

parfor j = 1:length(b)

[ x y z ] =load_func('my_variables.mat')

…

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

function [ x y z ] =load_func(file)

load( file );

[Walter Roberson]

Always use the function form of load, which returns a structure with one field for every variable loaded. Then extract information from the struct into the variables.

function [ x y z ] =load_func(file)
load( file );

will not establish any connection between variables x y z stored in the file, and what is to be returned.

function [ x y z ] =load_func(file)
S = load( file );
x = S.x;
y = S.y;
z = S.z;