14. Scheduler Manager

14.1. Introduction

The scheduler manager ...

The directives provided by the scheduler manager are:

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