Chapter 37. Setting the priority for a process with library calls

Real-time processes use a different set of library calls to control policy and priority. The following library calls are used to set the priority of non-real-time processes.

These functions adjust the nice value of a non-real-time process. The nice value serves as a suggestion to the scheduler on how to order the list of ready-to-run, non-real-time processes to be run on a processor. The processes at the head of the list run before the ones further down the list.

The scheduler policy and other parameters can be set using the sched_setscheduler() function. Currently, real-time policies have one parameter, sched_priority. This parameter is used to adjust the priority of the process.

The sched_setscheduler() function requires three parameters, in the form: sched_setscheduler(pid_t pid, int policy, const struct sched_param *sp);.

If the process ID is zero, the sched_setscheduler() function acts on the calling process.

The following code excerpt sets the scheduler policy of the current process to the SCHED_FIFO scheduler policy and the priority to 50:

struct sched_param sp = { .sched_priority = 50 };
int ret;

ret = sched_setscheduler(0, SCHED_FIFO, &sp);
if (ret == -1) {
  perror("sched_setscheduler");
  return 1;
}

The sched_setparam() function is used to set the scheduling parameters of a particular process. This can then be verified using the sched_getparam() function.

Unlike the sched_getscheduler() function, which only returns the scheduling policy, the sched_getparam() function returns all scheduling parameters for the given process.

struct sched_param sp;
int ret;

ret = sched_getparam(0, &sp);
sp.sched_priority += 2;
ret = sched_setparam(0, &sp);

If this code were used in a real application, it would need to check the return values from the function and handle any errors appropriately.

The sched_setattr() function sets the scheduling policy and its associated attributes for an instance ID specified in PID. When pid=0, sched_setattr() acts on the process and attributes of the calling thread.