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Parallel for loop: Using broadcasting array variable values as indexes

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Good day.
I have recently aquired the parallel computing toolbox to speed up some of our applications. I have run into the following challenge and hope that someone can give me advice on this.
topNodes = [2 3 4 5 6 7];
bottomNodes = [1 2 3 4 5 6];
polarity = [1 1 -1 1 1 1];
nodes = max([topNodes bottomNodes]);
T = zeros(numel(polarity),nodes);
for i = 1:numel(polarity);
if polarity(i) < 0
T(i,bottomNodes(i)) = -1;
T(i,topNodes(i)) = 1;
else
T(i,bottomNodes(i)) = 1;
T(i,topNodes(i)) = -1;
end
end
Now, if I use a parfor loop, it complains. Some assistance will be much appreciated.
Kind regards,
Barend.

Risposta accettata

Sarah Wait Zaranek
Sarah Wait Zaranek il 14 Nov 2011
For parfor loops, when you index into a sliced variables, restrictions are placed on the first-level variable indices. This allows parfor to easily distribute the right part of the variable to the right workers. One of these first-level indices must be the loop counter variable or the counter variable plus or minus a constant. Every other first-level index must be a constant, a non-loop counter variable, a colon, or an end.
I explain this more in the blog post where you also left this question.
See a possible fix below.
topNodes = [2 3 4 5 6 7];
bottomNodes = [1 2 3 4 5 6];
polarity = [1 1 -1 1 1 1];
nodes = max([topNodes bottomNodes]);
T = zeros(numel(polarity),nodes);
parfor i = 1:numel(polarity);
myData = T(i,:);
if polarity(i) < 0
myData(i,bottomNodes(i)) = -1;
myData(i,topNodes(i)) = 1;
else
myData(i,bottomNodes(i)) = 1;
myData(i,topNodes(i)) = -1;
end
T(i,:) = myData;
end
  3 Commenti
Sarah Wait Zaranek
Sarah Wait Zaranek il 15 Nov 2011
Yes, there is an overhead to pass the data to and from the workers - so for code that takes seconds to run - there isn't often a speedup. Also, you are correct that vectorization often is a better choice then de-vectorizing and using parfor.
Konrad Malkowski
Konrad Malkowski il 17 Nov 2011
You should also try running the script as a function. In general MATLAB performance is better with functions that with scripts, as scripts are interpreted one line at a time.

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