Assignin in parfor loop

Hello everybody,

i have a problem with assignin in a parfor loop. For that I made a short example which explains the problem:

for i = 1:10
    beta = i;
    disp(beta);
end

parfor pi = 1:10
    assignin('base','beta',pi);
    disp(beta)
end

In the first loop beta gets values from 1 to 10, but in the parfor-loop beta always has value of 10. Why does this happen? I wanted to assignin for a variable as a input for simulink simulations.

Many thanks in advance,

Greetings

Sascha

[Jan]

I cannot image a situation, in which assignin is useful inside a parfor loop.

[Sascha]

I tried to run multiple simulink simulations in parallel, as I have seen it in: http://blogs.mathworks.com/seth/2010/10/17/parallel-computing-with-simulink-running-thousands-of-simulations/#11

Answer 1

[Walter Roberson]

Doing an assignin('base') is going to change the value in the base workspace, but code executed in parallel is executed in a non-base workspace (because a new process if formed.)

I am surprised assignin() is allowed in a parfor. If the value being assigned varies from loop to loop, the result would be entirely dependent on the order the loops ended up being executed, which is not determinate in parfor()

Answer 2

[Edric Ellis]

When you run your code at the command-line, all of the FOR loop body is run in the base workspace - whereas the PARFOR body is not. So, your ASSIGNIN statement is operating correctly, but the second line of your PARFOR loop body is not running in the base workspace - so it actually sees the value of beta set by the prior FOR loop. If you change your PARFOR loop to:

parfor pi = 1:10
  assignin('base','beta',pi);
  disp(evalin('base', 'beta'));
end

You should get the expected behaviour. Simulink models should also correctly see the value that you assign.

[Sascha]

Thanks for your answer. I already tried it with a simulink model. But it doesn't work as I expected, so the next step was to try it with disp, to see what values arre assigned to beta.

I tried it with simulink as in the following code-fragment:

parfor pi = 1:n
    load_system('invertedPendulum');
    assignin('base','beta',pi);
    sim('invertedPendulum');
    outputsParfor(pi) = simout;
end

figure;
for j = 1:n 
    plot(outputsParfor(j));hold all;
end
hold off;

But in the plot I noticed that there were n-plots, but all were plotted with a value of p=n.