Scheduling policies

To meet the needs of various applications, BlackBerry 10 OS provides these scheduling policies:

• FIFO scheduling — SCHED_FIFO
• Round-robin scheduling — SCHED_RR
• Sporadic scheduling — SCHED_SPORADIC

Each thread in the system may run using any method. Scheduling methods are effective on a per-thread basis, not on a global basis for all threads and processes on a node.

Remember that these scheduling policies apply only when two or more threads that share the same priority are READY (i.e. the threads are directly competing with each other). If a higher-priority thread becomes READY, it immediately preempts all lower-priority threads.

In the following diagram, three threads of equal priority are READY. If Thread A blocks, Thread B will run.

A thread can call pthread_attr_setschedparam() or pthread_attr_setschedpolicy() to set the scheduling parameters and policy to use for any threads that it creates.

Although a thread inherits its scheduling policy from its parent thread, the thread can call pthread_setschedparam() to request to change the algorithm and priority applied by the kernel, or pthread_setschedprio() to change just the priority. A thread can get information about its current algorithm and policy by calling pthread_getschedparam() . Both these functions take a thread ID as their first argument; you can call pthread_self() to get the calling thread's ID. For example:

struct sched_param param;
int policy, retcode;

/* Get the scheduling parameters. */

retcode = pthread_getschedparam( pthread_self(), &policy, &param);
if (retcode != EOK) {
    printf ("pthread_getschedparam: %s.\n", strerror (retcode));
    return EXIT_FAILURE;
}

printf ("The assigned priority is %d, and the current priority is %d.\n",
        param.sched_priority, param.sched_curpriority);

/* Increase the priority. */

param.sched_priority++;

retcode = pthread_setschedparam( pthread_self(), policy, &param);
if (retcode != EOK) {
    printf ("pthread_setschedparam: %s.\n", strerror (retcode));
    return EXIT_FAILURE;
}

When you get the scheduling parameters, the sched_priority member of the sched_param structure is set to the assigned priority, and the sched_curpriority member is set to the priority that the thread is currently running at (which could be different because of priority inheritance).

Our libraries provide a number of ways to get and set scheduling parameters:

pthread_getschedparam() , pthread_setschedparam() , pthread_setschedprio()

These are your best choice for portability.

SchedGet() , SchedSet()

You can use these to get and set the scheduling priority and policy, but they aren't portable because they're kernel calls.

sched_getparam() , sched_setparam() , sched_getscheduler() , sched_setscheduler()

These functions are intended for use in single-threaded processes.

Note: Our implementations of these functions don't conform completely to POSIX. In multi-threaded applications, they get or set the parameters for thread 1 in the process pid, or for the calling thread if pid is 0. If you depend on this behavior, your code won't be portable. POSIX 1003.1 says these functions should return -1 and set errno to EPERM in a multi-threaded application.