Issue of classification of variables in a parfor.

I'd like to use parallel computing tool for massive calculations. I coded as below and tried to run it. and I got an error message like "The variable eval_val in a parfor cannot be classified". How do I solve this?

    % code
clc, clear all
parpool
xis=0.001;
mu=0.01:0.01:0.2;
gamma=0.9:0.01:1.1;
xit=0.001:0.01:0.4;
eval_val=zeros(length(gamma),length(xit));
ng=length(gamma);
nxt=length(xit);
parfor mm=1:length(mu)

    for gg=1:ng

        for tt=1:nxt

            M=[1+mu(mm), mu(mm);1,1];
            C=[2*xis,0;0, 2*gamma(gg)*xit(tt)];
            K=[1,0;0,gamma(gg)^2];

            iM=inv(M);
            A=[zeros(2),eye(2);-iM*K,-iM*C];
            B=[zeros(2,1);1;0];

            Q=lyap(A,4*xis*B*B');
            eval_val(gg,tt)=Q(1,1);

        end

    end

    [gamma_opt,xit_opt]=find(eval_val==min(min(eval_val)));

    gamma_opt(mm)=gamma(gamma_opt);
    xit_opt(mm)=xit(xit_opt);
end

Answer 1

[Jeff Miller]

I'm not sure I follow this, but I think there is a problem in these lines:

      [gamma_opt,xit_opt]=find(eval_val==min(min(eval_val))); 
      gamma_opt(mm)=gamma(gamma_opt);
      xit_opt(mm)=xit(xit_opt);

gamma_opt and xit_opt will be modified by all of the different processors in parallel, and that looks to MATLAB like the different processor outputs will be overwriting each other. Try using other variables as outputs of the find command, e.g.

        [gamma_opt_idx,xit_opt_idx]=find(eval_val==min(min(eval_val)));

and change the next two lines accordingly.

Also, you should probably pre-allocate gamma_opt and xit_opt to hold length(mu) outputs.

[Yonghun Lee]

Yes, you're right. But the problem of the classifying variable named 'eval_val' in a parfor is not still solved.

[Jeff Miller]

Move this

eval_val=zeros(length(gamma),length(xit));

inside the parfor loop so that each processor gets its own copy of this array. Then MATLAB will see that this is just a temporary working variable, not a variable that is shared by all processors.

[Yonghun Lee]

I'm surprised of your insight! It was just solved! I'd like to be familiar with the tool as you! Thanks a lot!

[Jeff Miller]

You are welcome. Please accept the answer if the problem is solved.