Why is my parfor loop running slower than a regular for loop
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Hello all, I have a 3D array that takes an SVD of the first two dimensions for the size of the 3rd dimension. This seems like it would be extreemly parallelizable to me so I tried to run it in a parfor loop but for some reason the computation is like 10x slower running the parfor vs the standard for loop. Any help will be much appriciated!!
im_vec = rand(4,4,1000);
U = zeros(size(im_vec,1),size(im_vec,1),size(im_vec,3));
sig = zeros(size(im_vec));
V = zeros(size(im_vec,2),size(im_vec,2),size(im_vec,3));
%%% For loop time %%%
tic
for n = 1:size(im_vec,3)
[U(:,:,n), sig(:,:,n),V(:,:,n)] = svd(im_vec(:,:,n));
end
toc
%%% Parfor loop time %%%
p = gcp('nocreate');
if isempty(p)
parpool;
end
tic
parfor n = 1:size(im_vec,3)
[U(:,:,n), sig(:,:,n),V(:,:,n)] = svd(im_vec(:,:,n));
end
toc
1 个评论
Walter Roberson
2023-12-19
On my system, if I expand the work by a factor of 100, then the regular for loop takes about 0.58 seconds and the parfor loop takes about 0.22 seconds the first time (and about 0.15 seconds after that.)
回答（1 个）
Christine Tobler
2023-12-19
0 个投票
The problem is likely that the cost of each iteration is still too small to warrant the start-up time of the parfor loop.
You could take a look at the pagesvd function, which applies the SVD to each page A(:, :, i) of the input array, just like what you're doing here. It uses threading on a lower level when the size of the array means we expect the threading to be worthwhile.
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2023-12-19
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