How to get the data from all the workers to a single worker inside SPMD loop

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Naveen kumar Elumalai 2019-6-30

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/469516-how-to-get-the-data-from-all-the-workers-to-a-single-worker-inside-spmd-loop

编辑： Naveen kumar Elumalai 2019-6-30

Question_matrix.pdf

Nc=3600;
spmd
             n1=numlabs;
             blocksize = ceil(Nc/n1);
             start = (labindex-1)*blocksize + 1;
             finish = min(Nc, start+blocksize-1);
         for n=finish:-1:start
                   A_sample=rand(600, 53)                                                                
                   index_cpu = n-start+1;
                   B_sample (:,:,index_cpu)=A_sample;  % each worker has its own copy of B_sample.
         end          
end      
B_collect=cat(3,B_sample{:}); % B_sample is concatenated (collected from all workers) to a single worker outside the SPMD loop

In the above code, consider 4 workers (numlabs) are used inside parallel loop

A_sample data is copied into B_sample inside SPMD.
After this, inorder to collect the B_sample data from all the workers to a single variable b_collect, we concatenate the data from all the workers outside the SPMD in B_collect . This concatenation takes a lot of time as Nc value increases (Time expensive)
In order to avoid the concatenation, I need to assign a variable say C_sample and make all the 4 workers to write to that variable C_sample inside SPMD (instead of writing in its own copy of variable). I think this might avoid the time expense.

I have two questions now :

How should I do the 3RD point i.e collect the results in C_sample variable inside SPMD (i.e Say C_collect is a global variable and each worker updates in that variable, which will avoid the concatenation issue)
As each worker (1,2,3,4) has its own copy of B_sample in a composite format, is there any faster way i can send the data of B_sample from workers(2,3,4) to worker 1 inside the SPMD?

I have attached the image and pdf of the same for a better understanding of the question.