Scheduling Control

The MP C compiler supports several pragmas that can be used in conjunction with the taskloop pragma to control the loop scheduling strategy for a given loop. The syntax for this pragma is:

#pragma MP taskloop schedtype (scheduling_type)

This pragma can be used to specify the specific scheduling_type to be used to schedule the parallelized loop. Scheduling_type can be one of the following:

• static

In static scheduling all the iterations of the loop are uniformly distributed among all the participating processors. Consider the following example:

#pragma MP taskloop maxcpus(4)
#pragma MP taskloop schedtype(static)
    for (i=0; i<1000; i++) {
...
}

In the above example, each of the four processors will process 250 iterations of the loop.

• self [(chunk_size)]

In self scheduling, each participating processor processes a fixed number of iterations (called the "chunk size") until all the iterations of the loop have been processed. The optional chunk_size parameter specifies the "chunk size" to be used. Chunk_size must be a positive integer constant, or variable of integral type. If specified as a variable chunk_size must evaluate to a positive integer value at the beginning of the loop. If this optional parameter is not specified or its value is not positive, the compiler will select the chunk size to be used. Consider the following example:

#pragma MP taskloop maxcpus(4)
#pragma MP taskloop schedtype(self(120))
for (i=0; i<1000; i++) {
...
}

In the above example, the number of iterations of the loop assigned to each participating processor, in order of work request, are:

120, 120, 120, 120, 120, 120, 120, 120, 40.

• gss [(min_chunk_size)]

In guided self scheduling, each participating processor processes a variable number of iterations (called the "min chunk size") until all the iterations of the loop have been processed. The optional min_chunk_size parameter specifies that each variable chunk size used must be at least min_chunk_size in size. Min_chunk_size must be a positive integer constant, or variable of integral type. If specified as a variable min_chunk_size must evaluate to a positive integer value at the beginning of the loop. If this optional parameter is not specified or its value is not positive, the compiler will select the chunk size to be used. Consider the following example:

#pragma MP taskloop maxcpus(4)
#pragma MP taskloop schedtype(gss(10))
for (i=0; i<1000; i++) {
...
}

In the above example, the number of iterations of the loop assigned to each participating processor, in order of work request, are:

250, 188, 141, 106, 79, 59, 45, 33, 25, 19, 14, 11, 10, 10, 10.

• factoring [(min_chunk_size)]

In factoring scheduling, each participating processor processes a variable number of iterations (called the "min chunk size") until all the iterations of the loop have been processed. The optional min_chunk_size parameter specifies that each variable chunk size used must be at least min_chunk_size in size. Min_chunk_size must be a positive integer constant, or variable of integral type. If specified as a variable min_chunk_size must evaluate to a positive integer value at the beginning of the loop. If this optional parameter is not specified or its value is not positive, the compiler will select the chunk size to be used. Consider the following example:

#pragma MP taskloop maxcpus(4)
#pragma MP taskloop schedtype(factoring(10))
for (i=0; i<1000; i++) {
...
}

In the above example, the number of iterations of the loop assigned to each participating processor, in order of work request, are:

125, 125, 125, 125, 62, 62, 62, 62, 32, 32, 32, 32, 16, 16, 16, 16, 10, 10, 10, 10, 10, 10.