Usage of sturctures inside parfor!!

Hi,

I have a code which I am using to simulate a model 'n' number of times using parallel computation toolbox. Some of the inputs to the model are in the form of "structures(a.b)" and " multiple structures(a.b(c))". I am loading the input through workspace. The code simulates without any errors in normal for loop. But when I change the for to parfor loop the code analyser shows an error that " Valid indices are restricted for this input inside the parfor loop" for structures and "Field reference for multiple structure elements that is followed by more reference blocks is an error." for multiple structures. I can change the name of structure and represent it as a variable but the problem is the same variable is used at different places when I integrate all my models. Hence I cant do that??

Is it possible to represent structures and multiple structures inside parfor loop??

For example:

structures = a.b;

multiple structures = a.b(c);

[Edric Ellis]

Please could you post a simple self-contained example of something you'd like to run but doesn't work in PARFOR?

[Abhishek M]

Hi Edric,

Please find the below code.

model = 'a_demo_model'; %%%My model %%%
numCases=100;
parfor j=1:numCases 
  S=load('a_demo_WS');%%%Loading of input to workspace through              %%%"demo_WS.mat"
  ip=S.ip; %%%Normal input %%%
  a.b=S.a.b; %%%Normal Structure input %%%
  a.d(fg)=S.a.d(fg); %%%Multiple Structure input %%%
  load_system(model) 
  assignin('base','ip',ip)
  assignin('base','a.b',a.b) 
  assignin('base','c.d(fg)',c.d(fg))
  simout(j)= sim(model,'SimulationMode','normal');  
end

I am running 'a_demo_model' using this code. But I have 2 more models to be integrated. All the 3 models which i have not yet integrated uses the same input 'b(normal structure)' and 'd(fg)(multiple structure)'. But parfor is throwing these( " Valid indices are restricted for this input inside the parfor loop" for structures and "Field reference for multiple structure elements that is followed by more reference blocks is an error." for multiple structures. ") errors if I define these inputs as the syntax specified in the above code.

I hope I am clear with this to you or else we can discuss more for further clarifications.

Answer 1

[Edric Ellis]

A simpler reproduction of the problem is shown below:

    parfor idx = 1:2
        a.x = 1;
        a.y = 2;
    end

The problem here is that technically the iterations of the loop cannot be proven to be order-independent (even though we can clearly see that they are). The reasons for this are a little obscure, and are to do with how structure dot-referencing works - basically, MATLAB treats "a.x" as a indexing request into the variable "a". So, in the loop above, you're indexing into "a" twice, but not "slicing" it - and in a FOR loop, the value of "a.y" would be available at the start of the next iteration of the loop, thus making it technically not order-independent.

In this case, the fix is simply to ensure that you assign a complete new value to "a" each time round the loop - then MATLAB will be able to see that "a" is a temporary variable that needs to live only for the duration of the iteration. Like so:

    parfor idx = 1:2
        a = [];
        a.x = 1;
        a.y = 2;
    end

Also, note that the ASSIGNIN you posted is incorrect. You probably need

    assignin('base', 'a', a)

[Abhishek M]

Thanx Edric,

your change worked for normal structure. But I am not able to solve for multiple structure..

parfor idx = 1:2
a=[];
a.d(fg)=2;
end

the error thrown is " An UndefinedFunction error was thrown on the workers for 'fg'. This may be because the file containing 'fg' is not accessible on the workers. Specify the required files to this MATLAB pool using the matlabpool addAttachedFiles command. See the documentation for matlabpool for more details."

[Edric Ellis]

What is "fg" supposed to be?

[Abhishek M]

Hi Edric,

"fg" is an array input. the representation of multiple sturucture "a.d(fg)" is like inside the main structure 'a' the structure 'd' of value 'fg'.

for example: fg=1.

then the value d(1) will be stored in structure 'a'.

[Abhishek M]

Hi Edric, my mistake sorry for mentioning previously, my syntax of structures and multiple structures doesn't work in normal for loop also...