How to access the indexes of elements of a replicated matrix 'A' inside an spmd block?

Suppose that I have a code as given below

spmd
    % assume numlabs to be 4
    nx = 3; ny = 3;
    A = zeros(nx,ny);
end

We know that 'A' is a matrix of size 3x3 which is locally present on each worker. How to share the rightmost column of matrix 'A' present on worker 1 with the matrix 'A' present on the worker 2 and so on. Also, if (i,j) represents the index of an element of matrix 'A' on the border of worker 1. How to access the element next to the element (i,j) which is present on the worker 2?

I have tried building a codistributed array from the replicated array 'A' but the resulting code was running very slow. Hence it of no use to me.

[Matt J]

I have tried building a codistributed array from the replicated array 'A' but the resulting code was running very slow

The sharing that you describe forces the workers to synchronize. Doing this too often can definitely slow things down.

Answer 1

[Matt J]

How to share the rightmost column of matrix 'A' present on worker 1 with the matrix 'A' present on the worker 2 and so on.

One way is to use gcat, e.g.,

parpool('local',4);

spmd
    
    A=labindex*eye(3);

 
      column3=gcat(A(:,3));
        
      k=mod(labindex,numlabs)+1;
    
    B=[A,column3(:,k)*10];  

end

>> C=cat(3,B{:})

C(:,:,1) =

     1     0     0     0
     0     1     0     0
     0     0     1    20


C(:,:,2) =

     2     0     0     0
     0     2     0     0
     0     0     2    30


C(:,:,3) =

     3     0     0     0
     0     3     0     0
     0     0     3    40


C(:,:,4) =

     4     0     0     0
     0     4     0     0
     0     0     4    10

Also, if (i,j) represents the index of an element of matrix 'A' on the border of worker 1. How to access the element next to the element (i,j) which is present on the worker 2?

Similarly,

i=1;j=2;
A0=10*reshape(1:9,3,3),
spmd
    
    A=A0+labindex;
    
    B=A;
    B(:)=circshift(B(:),1-labindex);
    
    ij=gcat(B(i,j));    
    next=mod(labindex,numlabs)+1;
    ij=ij(next);
end

>> C=cat(3,A{:}), values=[ij{:}]

C(:,:,1) =

    11    41    71
    21    51    81
    31    61    91


C(:,:,2) =

    12    42    72
    22    52    82
    32    62    92


C(:,:,3) =

    13    43    73
    23    53    83
    33    63    93


C(:,:,4) =

    14    44    74
    24    54    84
    34    64    94


values =

    52    63    74    41