When threads are not present, this routine has no effect.

    This routine notifies the thread scheduler that the current
    thread is willing to release its processor to other threads of
    equivalent or greater scheduling precedence. (A thread generally
    will release its processor to a thread of a greater scheduling
    precedence without calling this routine.) If no other threads of
    equivalent or greater scheduling precedence are ready to execute,
    the thread continues.

    This routine can allow knowledge of the details of an application
    to be used to improve its performance. If a thread does not call
    tis_yield(), other threads may be given the opportunity to run
    at arbitrary points (possibly even when the interrupted thread
    holds a required resource). By making strategic calls to tis_
    yield(), other threads can be given the opportunity to run when
    the resources are free. This improves performance by reducing
    contention for the resource.

    As a general guideline, consider calling this routine after a
    thread has released a resource (such as a mutex) which is heavily
    contended for by other threads. This can be especially important
    if the program is running on a uniprocessor machine, or if the
    thread acquires and releases the resource inside a tight loop.

    Use this routine carefully and sparingly, because misuse can
    cause unnecessary context switching that will increase overhead
    and actually degrade performance. For example, it is counter-
    productive for a thread to yield while it holds a resource that
    the threads to which it is yielding will need. Likewise, it is
    pointless to yield unless there is likely to be another thread
    that is ready to run.