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Utilising Multicore on Linux Matlab 7.11 64-bit

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Sanjay Manohar
Sanjay Manohar on 30 Mar 2011
I'm unable to get multicore working.
I am running the following in a script:
function partest
X=ones(10000);
for(a=1:100);
X.*X;
end
This is what my System Monitor shows: http://smanohar.com/matlab-multicore.png. As you can see, only one core is being used, despite the operation being element-wise!
I also get:
>> maxNumCompThreads
ans =
6
My system is:
Matlab: 7.11.0.584 R2010b, 64-bit (glnxa64)
OS: 64-bit Ubuntu 10.04, Kernel 2.6.32-29
CPU: Intel Xeon W3580, 6-core each @3.33 GHz
Why isn't it using all 6 CPUs? I thought that element-wise multiplication was parallellised?
Incidentally, Parallel toolbox is also installed. Under Parallel/Configurations Manager/local, when I press Start validation, the System monitor shows all cores are used for "Parallel Job" and "Matlabpool".

Accepted Answer

jiro
jiro on 5 Apr 2011
Element-wise matrix operations are multithreaded by default in your MATLAB version. In fact, this has been true since R2008a.
For your problem, it seems like X.*X is too small of a computation to see the multithreaded effect. From your System Monitor image, the whole computation took about 20 seconds, which means that each loop (X.*X) is taking only about 0.2 seconds, so that's not long enough to see the effect. Note that for-loops are not multithreaded. To see the effect, you need to be looking at a long (single) computation. Try something like
X=ones(10000);
sqrt(X).^sqrt(X).^sqrt(X).^sqrt(X).^sqrt(X).^sqrt(X)
or something like that.
  1 Comment
Sanjay Manohar
Sanjay Manohar on 13 Apr 2011
Aha! wonderful, thanks.
I tried a longer operation, as you suggested,
and it did indeed use all my CPUs!

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More Answers (1)

Murugavel Sivagnanam
Murugavel Sivagnanam on 31 Mar 2011
MALTAB by default is a single threaded process. Though you have multicore process, it does not use multiple cores to perform calculations.
Also the functions that you have written does not use any of the features of Parallel Computing Toolbox. Refer to the following documentation for more information on using the parallel computing toolbox.
  3 Comments
jiro
jiro on 5 Apr 2011
Correction on this answer. In R2010b (since R2008a), MATLAB automatically multi-threads certain operations (linear algebra and element-wise computations).

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