Why does my fft2 only use 1-3 instead all my 8 cores?

I am running Matlab(R2015b) 64bit and I am doing a simulation where most of the computational time goes to fft2 and ifft2. The documentation reads

'[...] numerical functions such as fft, […] are multithreaded in MATLAB. Multithreaded computations have been on by default in MATLAB since Release 2008a. These functions automatically execute on multiple computational threads in a single MATLAB session, allowing them to execute faster on multicore-enabled machines.'

But obviously the computer I am using can't use all of its cores (see screenshots).

A minimal example of my code looks like this:

Nx=64;
Ny=2^18;
psi=rand(Nx,Ny);
psik=zeros(Nx,Ny);

fftw('planner','patient');
for i=1:100
    psik(:,:)=fftshift(fft2(psi(:,:)));
end

When I increase the size of the bigger dimension by a factor 2, the problem gets even more obvious:

By searching in the internet I have only found someone with the same question. But his question was not answered ( http://de.mathworks.com/matlabcentral/answers/258370-how-to-run-fft-on-multi-core-without-parallel-computational-toolbox ). What can I do to fully use all available cores?

Regards

Tobias Nitschke

[Tobias Nitschke]

I have directly mailed at the support and got an answer from there. After some more testing I relized that my computer uses a little more than one core. The graphs shown at my graphics probably result from using the beginning of fftw. When I increase the smaller dimension my computer uses more CPU. Anyway I still haven't fully understood why the computer does not use its full power (all cores).

Answer 1

[Ahmed Rashid]

You can set the maximum number of threads using the command

maxNumCompThreads(number_of_threads)

If you want to see how many threads Matlab creates, you can type

maxNumCompThreads

By default it is equal to the number of cores that you have.