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Distribute Arrays and Run SPMD

Distributed Arrays

The workers in a parallel pool communicate with each other, so you can distribute an array among the workers. Each worker contains part of the array, and all the workers are aware of which portion of the array each worker has.

Use the distributed function to distribute an array among the workers:

M = magic(4) % a 4-by-4 magic square in the client workspace
MM = distributed(M)

Now MM is a distributed array, equivalent to M, and you can manipulate or access its elements in the same way as any other array.

M2 = 2*MM;  % M2 is also distributed, calculation performed on workers
x = M2(1,1) % x on the client is set to first element of M2

Single Program Multiple Data (spmd)

The single program multiple data (spmd) construct lets you define a block of code that runs in parallel on all the workers in a parallel pool. The spmd block can run on some or all the workers in the pool.

spmd     % By default creates pool and uses all workers
    R = rand(4);
end

This code creates an individual 4-by-4 matrix, R, of random numbers on each worker in the pool.

Composites

Following an spmd statement, in the client context, the values from the block are accessible, even though the data is actually stored on the workers. On the client, these variables are called Composite objects. Each element of a composite is a symbol referencing the value (data) on a worker in the pool. Note that because a variable might not be defined on every worker, a Composite might have undefined elements.

Continuing with the example from above, on the client, the Composite R has one element for each worker:

X = R{3};  % Set X to the value of R from worker 3.

The line above retrieves the data from worker 3 to assign the value of X. The following code sends data to worker 3:

X = X + 2;
R{3} = X; % Send the value of X from the client to worker 3.

If the parallel pool remains open between spmd statements and the same workers are used, the data on each worker persists from one spmd statement to another.

spmd
    R = R + labindex  % Use values of R from previous spmd.
end

A typical use for spmd is to run the same code on a number of workers, each of which accesses a different set of data. For example:

spmd
    INP = load(['somedatafile' num2str(labindex) '.mat']);
    RES = somefun(INP)
end

Then the values of RES on the workers are accessible from the client as RES{1} from worker 1, RES{2} from worker 2, etc.

There are two forms of indexing a Composite, comparable to indexing a cell array:

  • AA{n} returns the values of AA from worker n.

  • AA(n) returns a cell array of the content of AA from worker n.

Although data persists on the workers from one spmd block to another as long as the parallel pool remains open, data does not persist from one instance of a parallel pool to another. That is, if the pool is deleted and a new one created, all data from the first pool is lost.

For more information about using distributed arrays, spmd, and Composites, see Distributed Arrays.