Does Parfor make a difference without Parallel Computing toolbox

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cr on 11 Aug 2011
Commented: Walter Roberson on 23 Feb 2017
Hi, I've been running some experiment to see how parfor behaves. I dont have parallel computing toolbox (PCT) so I cant create workers. I found parfor giving no time-advantage. Is that expected? I'm on a MAC 64-bit core2duo machine.
Also, sometimes on PC (32bit XP core2duo) I open up task manager to see only one of the cores is being used while matlab is busy computing. Why is that? Wouldn't it be faster using both? Can I someway distribute jobs to different cores using parfor without PCT?
Thanks, cK.

Accepted Answer

Titus Edelhofer
Titus Edelhofer on 11 Aug 2011
just adding a comment for parfor: parfor does something "meaningful" for you in the sense of distributing execution only when you have the PCT. The reason you can use parfor within pure MATLAB at all is that you don't have to change the code when running on MATLAB without PCT... So: the answer is "yes, that is expected" ;-).

More Answers (4)

Thomas on 11 Oct 2011
Parfor without having a matlabpool open in slower than the comparative for loop so I guess it should be doing something different.
tic parfor i=1:10000*1024
A(i) = sin(i*2*pi/1024);
end toc
Output: Elapsed time is 25.464860 seconds.
Same code without the parfor
tic for i=1:10000*1024
A(i) = sin(i*2*pi/1024);
end toc
Output: Elapsed time is 1.121148 seconds.
However with the matlabpool open the same parfor loop was much faster matlabpool open local 8
>> MyWave
Elapsed time is 4.462031 seconds.
(FYI I have a dual processor quad core machine with 16GB of ram)

Friedrich on 11 Aug 2011
parfor will run the loop in the reverse order if you dont have Parallel Computing Toolbox.
Konrad Malkowski
Konrad Malkowski on 17 Nov 2011
As far as I can remember (2008b or so), PARFOR always run the loop in reverse order.
This is by design.

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Arnaud Miege
Arnaud Miege on 11 Aug 2011
I think that if you don't have the Parallel Computing Toolbox, parfor will behave as a normal for loop. Similarly, you can only distribute jobs to different cores with the Parallel Computing Toolbox.
Walter Roberson
Walter Roberson on 23 Feb 2017
Hayatullahi Adeyemo:
No, not exactly. You can run two functions at the same time but not two scripts. Scripts violate transparency. I think you could run() the files that contain the scripts because run() is actually a function call.
That said: the majority of the time when I see people asking to run two scripts at the same time, they want some kind of connection between the scripts that the parallel computing facility does not provide. For example there is no access to graphics in any worker of parfor or spmd.

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cr on 12 Aug 2011
Thanks for taking time to answer guys. More experiments corroborated what you said and like Friedrich commented, parfor makes the order reversed without PCT - as tested on 2010a.

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