Technical data

Advanced Features

This trade-off between response time and CPU usage can be adjusted with

the mp_blocktime(3f) call. mp_blocktime takes a single integer argument

that speciﬁes the number of times to spin before blocking. By default, it is set

to 10,000,000; this takes roughly 3 seconds. If called with an argument of 0,

the slave threads will not block themselves no matter how much time has

passed. Explicit calls to mp_block, however, will still block the threads.

This automatic blocking is transparent to the user’s program; blocked

threads are automatically unblocked when a parallel region is reached.

mp_numthreads, mp_set_numthreads

Occasionally, you may want to know how many execution threads are

available. mp_numthreads(3f) is a zero-argument integer function that

returns the total number of execution threads for this job. The count includes

the master thread.

mp_set_numthreads(3f) takes a single-integer argument. It changes the

default number of threads to the speciﬁed value. A subsequent call to

mp_setup will use the speciﬁed value rather than the original defaults. If the

slave threads have already been created, this call will not change their

number. It only has an effect when mp_setup is called.

mp_my_threadnum

mp_my_threadnum(3f) is a zero-argument function that allows a thread to

differentiate itself while in a parallel region. If there are n execution threads,

the function call returns a value between zero and n – 1. The master thread

is always thread zero. This function can be useful when parallelizing certain

kinds of loops. Most of the time the loop index variable can be used for the

same purpose. Occasionally, the loop index may not be accessible, as, for

example, when an external routine is called from within the parallel loop.

This routine provides a mechanism for those rare cases.