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Execute code in parallel on workers of parallel pool


spmd, statements, end
spmd(n), statements, end
spmd(m,n), statements, end


The general form of an spmd (single program, multiple data) statement is:


spmd, statements, end defines an spmd statement on a single line. MATLAB® executes the spmd body denoted by statements on several MATLAB workers simultaneously. The spmd statement can be used only if you have Parallel Computing Toolbox. To execute the statements in parallel, you must first open a pool of MATLAB workers using parpool or have your parallel prefences allow the automatic start of a pool.

Inside the body of the spmd statement, each MATLAB worker has a unique value of labindex, while numlabs denotes the total number of workers executing the block in parallel. Within the body of the spmd statement, communication functions for communicating jobs (such as labSend and labReceive) can transfer data between the workers.

Values returning from the body of an spmd statement are converted to Composite objects on the MATLAB client. A Composite object contains references to the values stored on the remote MATLAB workers, and those values can be retrieved using cell-array indexing. The actual data on the workers remains available on the workers for subsequent spmd execution, so long as the Composite exists on the client and the parallel pool remains open.

By default, MATLAB uses as many workers as it finds available in the pool. When there are no MATLAB workers available, MATLAB executes the block body locally and creates Composite objects as necessary.

spmd(n), statements, end uses n to specify the exact number of MATLAB workers to evaluate statements, provided that n workers are available from the parallel pool. If there are not enough workers available, an error is thrown. If n is zero, MATLAB executes the block body locally and creates Composite objects, the same as if there is no pool available.

spmd(m,n), statements, end uses a minimum of m and a maximum of n workers to evaluate statements. If there are not enough workers available, an error is thrown. m can be zero, which allows the block to run locally if no workers are available.

For more information about spmd and Composite objects, see Distribute Arrays and Run SPMD.


Perform a simple calculation in parallel, and plot the results:

  % build magic squares in parallel
  q = magic(labindex + 2);
for ii=1:length(q)
  % plot each magic square
  figure, imagesc(q{ii});

More About

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  • An spmd block runs on the workers of the existing parallel pool. If no pool exists, spmd will start a new parallel pool, unless the automatic starting of pools is disabled in your parallel preferences. If there is no parallel pool and spmd cannot start one, the code runs serially in the client session.

  • If the AutoAttachFiles property in the cluster profile for the parallel pool is set to true, MATLAB performs an analysis on an spmd block to determine what code files are necessary for its execution, then automatically attaches those files to the parallel pool job so that the code is available to the workers.

  • For information about restrictions and limitations when using spmd, see Run Single Programs on Multiple Data Sets.

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