SCHED_SETSCHEDULER

NAME

SYNOPSIS

#include <sched.h>

int sched_setscheduler(pid_t pid, int policy,


                       const struct sched_param *param);

int sched_getscheduler(pid_t pid);

struct sched_param {
    ...
    int sched_priority;
    ...
};

DESCRIPTION

SCHED_OTHER

the standard round-robin time-sharing policy;

SCHED_BATCH

for "batch" style execution of processes; and

SCHED_IDLE

for running very low priority background jobs.

The following "real-time" policies are also supported, for special time-critical applications that need precise control over the way in which runnable processes are selected for execution:

SCHED_FIFO

a first-in, first-out policy; and

SCHED_RR

a round-robin policy.

The semantics of each of these policies are detailed below.

sched_getscheduler() queries the scheduling policy currently applied to the process identified by pid. If pid equals zero, the policy of the calling process will be retrieved.  

Scheduling Policies

For processes scheduled under one of the normal scheduling policies (SCHED_OTHER, SCHED_IDLE, SCHED_BATCH), sched_priority is not used in scheduling decisions (it must be specified as 0).

Processes scheduled under one of the real-time policies (SCHED_FIFO, SCHED_RR) have a sched_priority value in the range 1 (low) to 99 (high). (As the numbers imply, real-time processes always have higher priority than normal processes.) Note well: POSIX.1-2001 only requires an implementation to support a minimum 32 distinct priority levels for the real-time policies, and some systems supply just this minimum. Portable programs should use sched_get_priority_min(2) and sched_get_priority_max(2) to find the range of priorities supported for a particular policy.

Conceptually, the scheduler maintains a list of runnable processes for each possible sched_priority value. In order to determine which process runs next, the scheduler looks for the non-empty list with the highest static priority and selects the process at the head of this list.

A process's scheduling policy determines where it will be inserted into the list of processes with equal static priority and how it will move inside this list.

All scheduling is preemptive: if a process with a higher static priority becomes ready to run, the currently running process will be preempted and returned to the wait list for its static priority level. The scheduling policy only determines the ordering within the list of runnable processes with equal static priority.  

SCHED_FIFO: First In-First Out scheduling

*

A SCHED_FIFO process that has been preempted by another process of higher priority will stay at the head of the list for its priority and will resume execution as soon as all processes of higher priority are blocked again.

*

When a SCHED_FIFO process becomes runnable, it will be inserted at the end of the list for its priority.

*

A call to sched_setscheduler() or sched_setparam(2) will put the SCHED_FIFO (or SCHED_RR) process identified by pid at the start of the list if it was runnable. As a consequence, it may preempt the currently running process if it has the same priority. (POSIX.1-2001 specifies that the process should go to the end of the list.)

*

A process calling sched_yield(2) will be put at the end of the list.

No other events will move a process scheduled under the SCHED_FIFO policy in the wait list of runnable processes with equal static priority.

A SCHED_FIFO process runs until either it is blocked by an I/O request, it is preempted by a higher priority process, or it calls sched_yield(2).  

SCHED_RR: Round Robin scheduling

SCHED_OTHER: Default Linux time-sharing scheduling

SCHED_BATCH: Scheduling batch processes

This policy is useful for workloads that are non-interactive, but do not want to lower their nice value, and for workloads that want a deterministic scheduling policy without interactivity causing extra preemptions (between the workload's tasks).  

SCHED_IDLE: Scheduling very low priority jobs

This policy is intended for running jobs at extremely low priority (lower even than a +19 nice value with the SCHED_OTHER or SCHED_BATCH policies).  

Privileges and resource limits

Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a ceiling on an unprivileged process's static priority for the SCHED_RR and SCHED_FIFO policies. The rules for changing scheduling policy and priority are as follows:

*

If an unprivileged process has a non-zero RLIMIT_RTPRIO soft limit, then it can change its scheduling policy and priority, subject to the restriction that the priority cannot be set to a value higher than the maximum of its current priority and its RLIMIT_RTPRIO soft limit.

*

If the RLIMIT_RTPRIO soft limit is 0, then the only permitted changes are to lower the priority, or to switch to a non-real-time policy.

*

Subject to the same rules, another unprivileged process can also make these changes, as long as the effective user ID of the process making the change matches the real or effective user ID of the target process.

*

Special rules apply for the SCHED_IDLE: an unprivileged process operating under this policy cannot change its policy, regardless of the value of its RLIMIT_RTPRIO resource limit.

Privileged (CAP_SYS_NICE) processes ignore the RLIMIT_RTPRIO limit; as with older kernels, they can make arbitrary changes to scheduling policy and priority. See getrlimit(2) for further information on RLIMIT_RTPRIO.  

Response time

Miscellaneous

Memory locking is usually needed for real-time processes to avoid paging delays; this can be done with mlock(2) or mlockall(2).

Since a non-blocking infinite loop in a process scheduled under SCHED_FIFO or SCHED_RR will block all processes with lower priority forever, a software developer should always keep available on the console a shell scheduled under a higher static priority than the tested application. This will allow an emergency kill of tested real-time applications that do not block or terminate as expected. See also the description of the RLIMIT_RTTIME resource limit in getrlimit(2).

POSIX systems on which sched_setscheduler() and sched_getscheduler() are available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>.  

RETURN VALUE

ERRORS

EINVAL

The scheduling policy is not one of the recognized policies, or param does not make sense for the policy.

EPERM

The calling process does not have appropriate privileges.

ESRCH

The process whose ID is pid could not be found.

CONFORMING TO

NOTES

Originally, Standard Linux was intended as a general-purpose operating system being able to handle background processes, interactive applications, and less demanding real-time applications (applications that need to usually meet timing deadlines). Although the Linux kernel 2.6 allowed for kernel preemption and the newly introduced O(1) scheduler ensures that the time needed to schedule is fixed and deterministic irrespective of the number of active tasks, true real-time computing was not possible up to kernel version 2.6.17.  

Real-time features in the mainline Linux kernel

patch-kernelversion-rtpatchversion

and can be downloaded from http://www.kernel.org/pub/linux/kernel/projects/rt/.

Without the patches and prior to their full inclusion into the mainline kernel, the kernel configuration offers only the three preemption classes CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY, and CONFIG_PREEMPT_DESKTOP which respectively provide no, some, and considerable reduction of the worst-case scheduling latency.

With the patches applied or after their full inclusion into the mainline kernel, the additional configuration item CONFIG_PREEMPT_RT becomes available. If this is selected, Linux is transformed into a regular real-time operating system. The FIFO and RR scheduling policies that can be selected using sched_setscheduler() are then used to run a process with true real-time priority and a minimum worst-case scheduling latency.  

BUGS

SEE ALSO

Programming for the real world - POSIX.4 by Bill O. Gallmeister, O'Reilly & Associates, Inc., ISBN 1-56592-074-0

The kernel source file Documentation/scheduler/sched-rt-group.txt (since kernel 2.6.25).  

COLOPHON