How to solve "parfor cannot be classified" issue?

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Xin Shen
Xin Shen 2019-10-4
评论： Walter Roberson 2019-11-1
Greetings,
I tried to convert my program with nested for-loops to parfor loop and got error "The variable z in a parfor cannot be classified". I read documents about "Troubleshoot Variables in parfor-Loops". But I don't totally understand it and even I tried different ways, the program still does not work.
Which rule I violate ( the slicing or nested for-loops )? And how to solve it, pack it as a function?
function xhat = eGram(Func,ParaVector,X0,scaling,flag)
Tol = 1E-7;
% gramian parameters
n = ParaVector(1);
k = ParaVector(2);
% initialization for T for observability
T(:,:,1) = eye(n,n);
Cm = logspace(log10(0.01),log10(1),5) .';
e = eye(n,n);
s = length(Cm);
r = size(T,3);
xhat = zeros(n,n,r*s);
Ts = 0.1;
nu = 600;
[~,~,Y0] = Func(X0);
if scaling
Y0_ = scale(Y0,Y0,Tol);
else
Y0_ = Y0;
end
for l=1:r
for m=1:s
z = zeros(n,nu*k);
parfor i=1:n % !!! Here said variable z does not use
% apply perturbed initial condition
initvalue = X0 + Cm(m)*T(:,:,l)*e(:,i);
[t,X,Y] = Func(initvalue);
if scaling
Y_ = scale(Y,Y0,Tol);
else
Y_ = Y;
end
if flag==0
for iii = 1:k
z(i,nu*(iii-1)+1:nu*iii) = (Y_(:,iii) - Y0_(1,iii))';
end
else
for iii = 1:k
z(i,nu*(iii-1)+1:nu*iii) = (Y_(:,iii) - Y0_(:,iii))';
end
end
end
chsi = z*z';
xhat(:,:,(l-1)*s+m) = 1/(r*s*Cm(m)^2)*Ts*T(:,:,l)*chsi*T(:,:,l)';
if any(isnan(xhat))
disp(['l is ' num2str(l)]);
disp(['m is ' num2str(m)]);
disp(['i is ' num2str(i)]);
end
end
end
end

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采纳的回答

Walter Roberson
Walter Roberson 2019-10-4
zt = zeros(1, nu*k);
for iii = 1:k
zt(1,nu*(iii-1)+1:nu*iii) = (Y_(:,iii) - Y0_(1,iii))';
end
z(ii, :) = zt;
You can probably vectorize. It looks like it would be something like
zt = Y_ - Y0_(1,:);
z(ii,:) = zt(:);
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Xin Shen
Xin Shen 2019-10-31
Thank you! I followed Walter Roberson's suggestion and solved it.
I have read these links you sent to me, but frankly speaking, these pages are the most confusing help files of Matlab. Even I read them several times, when I write my programs based on parallel computation and I still frequently get errors. Is there any good books or websites explainning these variable classification issues clearly?
Thank you!
Walter Roberson
Walter Roberson 2019-11-1
I recomend keeping it simple:
  1. Try making the parfor index the last index into any variable. In particular, avoid making it the first index, as first index will give you the least efficient memory transfer.
  2. For each variable that you are reading from, or which you are doing a read/modify/write for, extract all of the variable indexed by the parfor value into a temporary variable, so what you get in the temporary variable is just the parts that have to do with the parfor index
  3. For each variable that you are writing into but not as a read/modify/write cycle, initialize a temporary variable that is the size of the data related to the slice indexed by the parfor index.
  4. Manipulate the temporary variables as needed, including writing to them
  5. For each read/modify/write variable, assign the temporary variable back over the slice indexed by the parfor index; for each variable writing into but not read/modify/write, assign the temporary variable into the output variable indexed by the parfor index.
If you have some very large variables you might need to modify this sequence. If you have particular need for distributed variables instead of sliced variables, then you might need to modify this sequence.

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