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Programming Tips

MATLAB Path

All workers executing an spmd statement must have the same MATLAB® search path 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 workers, if possible. For more information, see the parpool reference page. When the workers are running on a different platform than the client, use the function pctRunOnAll to properly set the MATLAB path on all workers.

Error Handling

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

Errors and warnings produced on workers are annotated with the worker ID (labindex) 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 workers. 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 to a variable.

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 directly contain another spmd. However, it can call a function that contains another spmd statement. The inner spmd statement does not run in parallel in another parallel pool, but runs serially in a single thread on the worker running its containing function.

Nested parfor-Loops

The body of a parfor-loop cannot contain an spmd statement, and an spmd statement cannot contain a parfor-loop.

Break and Return Statements

The body of an spmd statement cannot contain break or return statements.

Global and Persistent Variables

The body of an spmd statement cannot contain global or persistent variable declarations.

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