Error while using fmincon in parallel computing mode

Question

I am using UseParallel command to go for parallel computing in fmincon as normal computation is taking a lot of time. But while using

np = 6
Cost = @(u)cost_min( u, CNT, u_in, y_in, Last );
options = optimoptions( 'fmincon', 'UseParallel', true );
   [ufinal,fval,exitflag,output,grad] = fmincon( Cost, u, A, B, [], [], lb, ub, [], options );

function Cost = cost_min(u,CNT,u_in,y_in,Last)
global np 

    for i = 1
    Costi(i) = (y(i) - yref(i))'*Q(i,i)*(y(i) - yref(i)) + (u(i)- u0)'*R(i,i)*(u(i)- u0);
     end
    for i = 2:np
    Costi(i) = (y(i) - yref(i))'*Q(i,i)*(y(i) - yref(i)) + (u(i)- u(i-1))'*R(i,i)*(u(i)- u(i-1));
    end

 Cost = sum(Costi);

Simulink is showing the error

Size vector should be a row vector with real elements.

While without the UseParallel option, the simulation is working fine.

Answer 1

The problem is most likely due to your use of a global variable, which should be avoided in general and specifically if using the Parallel Processing functionality.

See this question on globals and parfor, and the last section of the documentation Troubleshoot Variables in parfor-Loops.