MATLAB Answers

0

Matrix multiply slices of 3d Matricies

Asked by Dan Ryan on 5 Feb 2013
Given two 3d matricies, A and B with
size(A) = (n, m, k)
and
size(B) = (m, p, k)
perform matrix multiplications on each slice obtained by fixing the last index, yielding a matrix C with
size(C) = (n, p, k).
To clarify, we would have
C(:, :, 1) = A(:, :, 1)*B(:, :, 1), ..., C(:, :, k) = A(:, :, k)*B(:, :, k).
I need to do this with gpuArrays in the most efficient manner possible.

  0 Comments

Sign in to comment.

3 Answers

Jill Reese 님의 답변 9 Sep 2013
 채택된 답변

If you have MATLAB R2013b, you can use the new gpuArray pagefun function like so:
C = pagefun(@mtimes, A, B);

  1 Comment

Brilliant! Thanks!

Sign in to comment.


James Tursa 님의 답변 14 Feb 2013
James Tursa 님이 편집함. 14 Feb 2013

If you are not restricted to gpuArrays you can do this:
C = mtimesx(A,B);
The MTIMESX function passes pointers to the slice data to BLAS library functions in the background, so it is pretty fast. You can find MTIMESX here:
MTIMESX is not yet multi-threaded across the third dimension (but an update is in the works). A nD matrix multiply multi-threaded on the third dimension called MMX can also be used:
C = MMX('mult', A, B);
MMX can be found here:

  1 Comment

great suggestion, I will keep an eye on this project

Sign in to comment.


Azzi Abdelmalek 님의 답변 5 Feb 2013
Azzi Abdelmalek 님이 편집함. 5 Feb 2013

n=3;
m=4;
k=5;
p=2;
A=rand(n,m,k)
B=rand(m,p,k)
C=zeros(n,p,k)
for ii=1:k
C(:,:,ii)=A(:,:,ii)*B(:,:,ii)
end

  7 Comments

Big slowdown... for example:
A = gpuArray.rand(1000, 100, 100, 'single');
B = gpuArray.rand(1000, 100, 'single');
C = gpuArray.zeros(1000, 100, 100, 'single');
Compare
for idx = 1:100
C(:, :, idx) = A(:, :, idx).*B;
end
with
C = bsxfun(@times, A, B);
There is about a factor of 50 slowdown with the for loop.
bsxfun don't work with
n=3;
m=4;
k=5;
p=2;
A=rand(n,m,k)
B=rand(m,p,k)
C = bsxfun(@times, A, B);
Dan,
Can you elaborate on the sizes of m, n , k, and p that you are interested in? It would be useful to know a ballpark number for the size of problem you want to solve. Do you have many small page sizes, a few large pages, or something else?
Thanks,
Jill

Sign in to comment.