vectorised code is terribly slower
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Why is the vectorized version of simple local maxima detection code significantly slower (~2-3 times) than its for-loop version?
%ntest data
X = rand(100000,1000);
% findig local maxima over columns of X
% for-loop version
tic;
[I,J] = size(X);
Ind = false(I,J);
for j = 1:J
Ind(:,j) = diff( sign( diff([0; X(:,j); 0]) ) ) < 0;
end
toc
% vectorized version (~3 times slower than for-loop)
tic;
Ind_ = diff(sign(diff([zeros(1,J);X;zeros(1,J)],1,1)),1,1) < 0;
toc
% result identity test
isequal(Ind,Ind_)
6 Kommentare
Bruno Luong
am 9 Sep. 2019
It is possibly that the DIFF implementation on array does not access sequently memory in case of 2D array data, but row-by-row of the array, that might slow down.
I don't think the multi-threading is wrongly implemented.
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