How to parallel 4 works with PARFOR with a Core i3 in Matlab

Question

I have Matlab R2012b for Ubuntu 64 bits. I have a Intel Core i3 CPU M 330 @ 2.13GHz × 4.

I want to use parfor to parallelize 4 loops at same time. Because Intel Core i3 has 2 Cores and 4 Threads I use this code:

if matlabpool('size') == 0 % checking to see if my pool is already open
    matlabpool(4)
else
    matlabpool close
    matlabpool(4)
end

And I obtain the following error:

Error:

You requested a minimum of 4 workers, but the cluster "local" has the NumWorkers property set to allow a maximum of 2 workers. To run a communicating job on more workers than this (up to a maximum of 12 for the Local cluster), increase the value of the NumWorkers property for the cluster. The default value of NumWorkers for a Local cluster is the number of cores on the local machine.

Why? The default value of NumWorkers in my machine is 2 but if I can do 4 loops at the same time, how do I get it?

Answer 1

To increase the default NumWorkers, open the Cluster Profile Manager (Parallel->Manage Cluster Profiles). Pick the localprofile, click edit, and increase NumWorkers to the maximum possible value (in your case 4). Now it is possible to create a matlabpool with more workers than physical cores on your machine.

However, note that using more workers than cores might lead to decreased performance as compared to having the same number of workers as cores.

Answer 2

To programmatically change the value of NumWorkers from 2 to 4 of the local cluster profile, you can use:

myCluster = parcluster('local');
myCluster.NumWorkers = 4;  % 'Modified' property now TRUE
saveProfile(myCluster);    % 'local' profile now updated,
                           % 'Modified' property now FALSE