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Problem in Using Parallel Feature for a "For Loop"!

Asked by Maryam on 17 Jul 2018
Latest activity Commented on by Maryam on 17 Jul 2018
I have written a code for Newton Raphson which includes two "for loops" inside of each other. Basically, I have a 1000 nonlinear equations, which are in a function called "f_int". In the "for loop" I try to construct tangent stiffness of these 1000 equations by finite difference, which is as below:
f_int = internal_eqns1000(X);
for i=1:1000
for j=1:nvar
X(j)=X(j)+h;
f_intp1 = internal_eqns1000(X);
k_k(i,j)=(f_intp1(i)-f_int(i))/h;
X(j)=X(j)-h;
end
end
After the completion of tangent stiffness, another "X" will be created and the loop will run again until convergence. This for loop might be required to run more than 100 times, and each time it takes 30 minutes average. I would like to use parallel to decrease the computational timing, but I was not successful. The change I made is as below:
f_int = internal_eqns1000(X);
parfor i=1:1000
for j=1:nvar
X(j)=X(j)+h;
f_intp1 = internal_eqns1000(X);
k_k(i,j)=(f_intp1(i)-f_int(i))/h;
X(j)=X(j)-h;
end
end
But this does not work and gives me an error: "due to the way "X" is defined, parallel computation cannot be used here".
Please let me know how can I write the code such that it will be run with parallel feature. Thank you.

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1 Answer

Answer by OCDER
on 17 Jul 2018
Edited by OCDER
on 17 Jul 2018
 Accepted Answer

But I didn't realize Newton Raphson iterations can be done in parallel. Or are you doing independent NR calc for many situations?
X = zeros(1000, nvar);
f_int = internal_eqns1000(X);
parfor i=1:1000
for j=1:nvar
X(i,j)=X(i,j)+h;
f_intp1 = internal_eqns1000(X(i,j));
k_k(i,j)=(f_intp1(i)-f_int(i))/h;
X(i,j)=X(i,j)-h;
end
end

  11 Comments

Thank you so much. It is completely clear and fine. I just have a quick question about "number of workers in a parallel pool". It uses 4 workers for me automatically. How can I increase it to 1000 for example? Thanks again for your response.
You can't get more workers than the number of cores in your computer. To change the number of workers to less than maximum, you could do this:
NumCores = 2; %Want 2 cores
delete(gcp('nocreate'));
parpool(NumCores);

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