Convert Nested for-Loops to parfor

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POLLY
POLLY 2018-11-30
编辑： Matt J 2018-11-30
I want to speed up my script and try to replace nested for loop with parfor. I know that I can parallelize either of the nested loops, but I cannot run both in parallel.
I am not sure how can I use a temporary arrays here if I need to save only recovery matrix.
Thanks for any help!
That is the part of my script I am working on:
%%
resultX0 = zeros(length(n), length(q), ntrials, M, N);
resultY0 = zeros(length(n), length(q), ntrials, M, N);
resultG = zeros(length(n), length(q), ntrials, M, N);
for i=1:length(n)
for j=1:length(q)
for k=1:ntrials
[G,X0,Y0]=matrix_plantsubm(M,N,c,n(i),p,q(j));
resultX0(i,j,k,:,:)=X0;
resultY0(i,j,k,:,:)=Y0;
resultG(i,j,k,:,:)=G;
end
end
end
%%
recovery = zeros(length(n), length(q));
error = zeros(length(n), length(q));
resultX=zeros(length(n),length(q), ntrials, M, N);
resultY=zeros(length(n),length(q), ntrials, M, N);
parfor i=1:length(n)
for j=1:length(q)
% Set gamma as in paper.
gamma = 6/((q(j) - p)*n(i));
for k=1:1:ntrials
tic
[X,Y,Q, iter] = ADMMcopy(squeeze(resultG(i,j,k,:,:)),c, n(i),q(j), gamma,tau, opt_tol, verbose, maxiter);
resultX(i,j,k, :, :)=X;
resultY(i,j,k, :, :)=Y;
C=resultX-resultX0;
a=arrayfun(@frobeniusnorm,C).^2;
b=arrayfun(@frobeniusnorm,resultX0).^2;
if a(i,j,k)/b(i,j,k)<opt_tol
recovery(i,j)=recovery(i,j)+1;
error(i,j)=error(i,j)+a(i,j,k)/b(i,j,k);
else
recovery(i,j)=0;
toc
save('Sparse_case.mat','recovery', '-v7.3');
end
end
end
end

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回答（2 个）

Walter Roberson
Walter Roberson 2018-11-30
inside the parfor
trec = zeroes(1,length(q))
inside the for k
trec(1,j) = trec(1,j) + whatever
After the for j terminates
recovery(i,:)=trec;
You probably need something similar for error .
It would be better to rename error to avoid interfering with the important infrastructure function named error
Matt J
Matt J 2018-11-30
编辑：Matt J 2018-11-30
You can convert the first set of nested loops to a single parfor loop as below. The second set of nested-loops is non-parallelizable because the computation of C depends on all preceding iterations of the loops.
sz=[length(n), length(q)];
S=prod(sz);
resultX0 = zeros(S,ntrials, M, N);
resultY0 = zeros(S,ntrials, M, N);
resultG = zeros(S,ntrials, M, N);
parfor s=1:S
[i,j] = ind2sub(sz,s);
[G,X0,Y0]=matrix_plantsubm(M,N,c,n(i),p,q(j));
resultX0(s,:,:,:)=repmat(X0,1,ntrials,1,1);
resultY0(s,:,:,:)=repmat(Y0,1,ntrials,1,1);
resultG(s,:,:,:)=repmat(G,1,ntrials,1,1);
end
resultX0 = reshape(resultX,[sz ntrials M N]);
resultY0 = reshape(resultY0,[sz ntrials M N]);
resultG = reshape(resultG,[sz ntrials M N]);

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