Does Parfor make a difference without Parallel Computing toolbox

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cr
cr 2011-8-11
评论： Walter Roberson 2017-2-23
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.

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采纳的回答

Titus Edelhofer
Titus Edelhofer 2011-8-11
Hi,
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" ;-).
Titus

更多回答（4 个）

Thomas
Thomas 2011-10-11
Parfor without having a matlabpool open in slower than the comparative for loop so I guess it should be doing something different.
E.g.:
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
Friedrich 2011-8-11
Hi,
parfor will run the loop in the reverse order if you dont have Parallel Computing Toolbox.
  3 个评论
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Konrad Malkowski
Konrad Malkowski 2011-11-17
As far as I can remember (2008b or so), PARFOR always run the loop in reverse order.
This is by design.
Konrad Malkowski
Konrad Malkowski 2011-11-17
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 2011-8-11
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.
HTH,
Arnaud
  2 个评论
Hayatullahi Adeyemo
Hayatullahi Adeyemo 2017-2-23
Please I need clarification. Can I use Parallel Computing Toolbox to execute two different scripts at the same time on MATLAB? Hayatullah
Walter Roberson
Walter Roberson 2017-2-23
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
cr 2011-8-12
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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