14. Scheduler Manager¶

14.1. Introduction¶

The scheduler manager …

The directives provided by the scheduler manager are:

sched_get_priority_min - Get Minimum Priority Value
sched_get_priority_max - Get Maximum Priority Value
sched_rr_get_interval - Get Timeslicing Quantum
sched_yield - Yield the Processor

14.2. Background¶

14.2.1. Priority¶

In the RTEMS implementation of the POSIX API, the priorities range from the low priority of sched_get_priority_min() to the highest priority of sched_get_priority_max(). Numerically higher values represent higher priorities.

14.2.2. Scheduling Policies¶

The following scheduling policies are available:

SCHED_FIFO: Priority-based, preemptive scheduling with no timeslicing. This is equivalent to what is called “manual round-robin” scheduling.
SCHED_RR: Priority-based, preemptive scheduling with timeslicing. Time quantums are maintained on a per-thread basis and are not reset at each context switch. Thus, a thread which is preempted and subsequently resumes execution will attempt to complete the unused portion of its time quantum.
SCHED_OTHER: Priority-based, preemptive scheduling with timeslicing. Time quantums are maintained on a per-thread basis and are reset at each context switch.
SCHED_SPORADIC: Priority-based, preemptive scheduling utilizing three additional parameters: budget, replenishment period, and low priority. Under this policy, the thread is allowed to execute for “budget” amount of time before its priority is lowered to “low priority”. At the end of each replenishment period, the thread resumes its initial priority and has its budget replenished.

14.3. Operations¶

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14.4. Directives¶

This section details the scheduler manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

14.4.1. sched_get_priority_min - Get Minimum Priority Value¶

CALLING SEQUENCE:

#include <sched.h>
int sched_get_priority_min(
    int policy
);

STATUS CODES:

On error, this routine returns -1 and sets errno to one of the following:

EINVAL

The indicated policy is invalid.

DESCRIPTION:

This routine return the minimum (numerically and logically lowest) priority for the specified policy.

NOTES:

NONE

14.4.2. sched_get_priority_max - Get Maximum Priority Value¶

CALLING SEQUENCE:

#include <sched.h>
int sched_get_priority_max(
    int policy
);

STATUS CODES:

On error, this routine returns -1 and sets errno to one of the following:

EINVAL

The indicated policy is invalid.

DESCRIPTION:

This routine return the maximum (numerically and logically highest) priority for the specified policy.

NOTES:

NONE

14.4.3. sched_rr_get_interval - Get Timeslicing Quantum¶

CALLING SEQUENCE:

#include <sched.h>
int sched_rr_get_interval(
    pid_t            pid,
    struct timespec *interval
);

STATUS CODES:

On error, this routine returns -1 and sets errno to one of the following:

`ESRCH`	The indicated process id is invalid.
`EINVAL`	The specified interval pointer parameter is invalid.

DESCRIPTION:

This routine returns the length of the timeslice quantum in the interval parameter for the specified pid.

NOTES:

The pid argument should be 0 to indicate the calling process.

14.4.4. sched_yield - Yield the Processor¶

CALLING SEQUENCE:

#include <sched.h>
int sched_yield( void );

STATUS CODES:

This routine always returns zero to indicate success.

DESCRIPTION:

This call forces the calling thread to yield the processor to another thread. Normally this is used to implement voluntary round-robin task scheduling.

NOTES:

NONE