Programming Considerations :: Single Program Multiple Data (spmd) (Parallel Computing Toolbox™)

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Programming Considerations

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MATLAB Path Error Handling Limitations

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MATLAB Path

All labs executing an spmd statement must have the same MATLAB path configuration as the client, so that they can execute any functions called in their common block of code. Therefore, whenever you use cd, addpath, or rmpath on the client, it also executes on all the labs, if possible. For more information, see the matlabpool reference page. When the labs are running on a different platform than the client, use the function pctRunOnAll to properly set the MATLAB path on all labs.

Error Handling

When an error occurs on a lab during the execution of an spmd statement, the error is reported to the client. The client tries to interrupt execution on all labs, and throws an error to the user.

Errors and warnings produced on labs are annotated with the lab ID and displayed in the client's Command Window in the order in which they are received by the MATLAB client.

The behavior of lastwarn is unspecified at the end of an spmd if used within its body.

Limitations

Transparency

The body of an spmd statement must be transparent, meaning that all references to variables must be "visible" (i.e., they occur in the text of the program).

In the following example, because X is not visible as an input variable in the spmd body (only the string 'X' is passed to eval), it does not get transferred to the labs. As a result, MATLAB issues an error at run time:

X = 5;
spmd
    eval('X');
end

Similarly, you cannot clear variables from a worker's workspace by executing clear inside an spmd statement:

spmd; clear('X'); end

To clear a specific variable from a worker, clear its Composite from the client workspace. Alternatively, you can free up most of the memory used by a variable by setting its value to empty, presumably when it is no longer needed in your spmd statement:

spmd
    <statements....>
    X = []; 
end

Examples of some other functions that violate transparency are evalc, evalin, and assignin with the workspace argument specified as 'caller'; save and load, unless the output of load is assigned.

MATLAB does successfully execute eval and evalc statements that appear in functions called from the spmd body.

Nested Functions

Inside a function, the body of an spmd statement cannot make any direct reference to a nested function. However, it can call a nested function by means of a variable defined as a function handle to the nested function.

Because the spmd body executes on workers, variables that are updated by nested functions called inside an spmd statement do not get updated in the workspace of the outer function.

Anonymous Functions

The body of an spmd statement cannot define an anonymous function. However, it can reference an anonymous function by means of a function handle.

Nested spmd Statements

The body of an spmd statement cannot contain another spmd. However, it can call a function that contains another spmd statement. Be sure that your MATLAB pool has enough labs to accommodate such expansion.