SCHED

NAME

DESCRIPTION

API summary

sched_setscheduler(2)

Set the scheduling policy and parameters of a specified thread.

sched_getscheduler(2)

Return the scheduling policy of a specified thread.

sched_setparam(2)

Set the scheduling parameters of a specified thread.

sched_getparam(2)

Fetch the scheduling parameters of a specified thread.

sched_get_priority_max(2)

Return the minimum priority available in a specified scheduling policy.

sched_get_priority_min(2)

Return the maximum priority available in a specified scheduling policy.

sched_rr_get_interval(2)

Fetch the quantum used for threads that are scheduled under the "round-robin" scheduling policy.

sched_yield(2)

Cause the caller to relinquish the CPU, so that some other thread be executed.

sched_setaffinity(2)

(Linux-specific) Set the CPU affinity of a specified thread.

sched_getaffinity(2)

(Linux-specific) Get the CPU affinity of a specified thread.

sched_setattr(2)

Set the scheduling policy and parameters of a specified thread. This (Linux-specific) system call provides a superset of the functionality of sched_setscheduler(2) and sched_setparam(2).

sched_getattr(2)

Fetch the scheduling policy and parameters of a specified thread. This (Linux-specific) system call provides a superset of the functionality of sched_getscheduler(2) and sched_getparam(2).

Scheduling policies

For threads 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 threads always have higher priority than normal threads.) Note well: POSIX.1-2001 requires an implementation to support only 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 threads for each possible sched_priority value. In order to determine which thread runs next, the scheduler looks for the nonempty list with the highest static priority and selects the thread at the head of this list.

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

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

SCHED_FIFO: First in-first out scheduling

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

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

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

*

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

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

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

SCHED_RR: Round-robin scheduling

SCHED_DEADLINE: Sporadic task model deadline scheduling

A sporadic task is one that has a sequence of jobs, where each job is activated at most once per period. Each job also has a relative deadline, before which it should finish execution, and a computation time, which is the CPU time necessary for executing the job. The moment when a task wakes up because a new job has to be executed is called the arrival time (also referred to as the request time or release time). The start time is the time at which a task starts its execution. The absolute deadline is thus obtained by adding the relative deadline to the arrival time.

The following diagram clarifies these terms:

arrival/wakeup                    absolute deadline
     |    start time                    |
     |        |                         |
     v        v                         v
-----x--------xooooooooooooooooo--------x--------x---
              |<- comp. time ->|
     |<------- relative deadline ------>|
     |<-------------- period ------------------->|

When setting a SCHED_DEADLINE policy for a thread using sched_setattr(2), one can specify three parameters: Runtime, Deadline, and Period. These parameters do not necessarily correspond to the aforementioned terms: usual practice is to set Runtime to something bigger than the average computation time (or worst-case execution time for hard real-time tasks), Deadline to the relative deadline, and Period to the period of the task. Thus, for SCHED_DEADLINE scheduling, we have:

arrival/wakeup                    absolute deadline
     |    start time                    |
     |        |                         |
     v        v                         v
-----x--------xooooooooooooooooo--------x--------x---
              |<-- Runtime ------->|
     |<----------- Deadline ----------->|
     |<-------------- Period ------------------->|

The three deadline-scheduling parameters correspond to the sched_runtime, sched_deadline, and sched_period fields of the sched_attr structure; see sched_setattr(2). These fields express value in nanoseconds. If sched_period is specified as 0, then it is made the same as sched_deadline.

The kernel requires that:

    sched_runtime <= sched_deadline <= sched_period

In addition, under the current implementation, all of the parameter values must be at least 1024 (i.e., just over one microsecond, which is the resolution of the implementation), and less than 2^63. If any of these checks fails, sched_setattr(2) fails with the error EINVAL.

The CBS guarantees non-interference between tasks, by throttling threads that attempt to over-run their specified Runtime.

To ensure deadline scheduling guarantees, the kernel must prevent situations where the set of SCHED_DEADLINE threads is not feasible (schedulable) within the given constraints. The kernel thus performs an admittance test when setting or changing SCHED_DEADLINE policy and attributes. This admission test calculates whether the change is feasible; if it is not sched_setattr(2) fails with the error EBUSY.

For example, it is required (but not necessarily sufficient) for the total utilization to be less than or equal to the total number of CPUs available, where, since each thread can maximally run for Runtime per Period, that thread's utilization is its Runtime divided by its Period.

In order to fulfil the guarantees that are made when a thread is admitted to the SCHED_DEADLINE policy, SCHED_DEADLINE threads are the highest priority (user controllable) threads in the system; if any SCHED_DEADLINE thread is runnable, it will preempt any thread scheduled under one of the other policies.

A call to fork(2) by a thread scheduled under the SCHED_DEADLINE policy will fail with the error EAGAIN, unless the thread has its reset-on-fork flag set (see below).

A SCHED_DEADLINE thread that calls sched_yield(2) will yield the current job and wait for a new period to begin.  

SCHED_OTHER: Default Linux time-sharing scheduling

SCHED_BATCH: Scheduling batch processes

This policy is useful for workloads that are noninteractive, 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).  

Resetting scheduling policy for child processes

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ORing the SCHED_RESET_ON_FORK flag into the policy argument when calling sched_setscheduler(2) (since Linux 2.6.32); or

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specifying the SCHED_FLAG_RESET_ON_FORK flag in attr.sched_flags when calling sched_setattr(2).

Note that the constants used with these two APIs have different names. The state of the reset-on-fork flag can analogously be retrieved using sched_getscheduler(2) and sched_getattr(2).

The reset-on-fork feature is intended for media-playback applications, and can be used to prevent applications evading the RLIMIT_RTTIME resource limit (see getrlimit(2)) by creating multiple child processes.

More precisely, if the reset-on-fork flag is set, the following rules apply for subsequently created children:

*

If the calling thread has a scheduling policy of SCHED_FIFO or SCHED_RR, the policy is reset to SCHED_OTHER in child processes.

*

If the calling process has a negative nice value, the nice value is reset to zero in child processes.

After the reset-on-fork flag has been enabled, it can be reset only if the thread has the CAP_SYS_NICE capability. This flag is disabled in child processes created by fork(2).  

Privileges and resource limits

A thread must be privileged (CAP_SYS_NICE) in order to set or modify a SCHED_DEADLINE policy.

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

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If an unprivileged thread has a nonzero 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.

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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 thread can also make these changes, as long as the effective user ID of the thread making the change matches the real or effective user ID of the target thread.

*

Special rules apply for the SCHED_IDLE policy. In Linux kernels before 2.6.39, an unprivileged thread operating under this policy cannot change its policy, regardless of the value of its RLIMIT_RTPRIO resource limit. In Linux kernels since 2.6.39, an unprivileged thread can switch to either the SCHED_BATCH or the SCHED_NORMAL policy so long as its nice value falls within the range permitted by its RLIMIT_NICE resource limit (see getrlimit(2)).

Privileged (CAP_SYS_NICE) threads 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.  

Limiting the CPU usage of real-time and deadline processes

Since Linux 2.6.25, there are other techniques for dealing with runaway real-time and deadline processes. One of these is to use the RLIMIT_RTTIME resource limit to set a ceiling on the CPU time that a real-time process may consume. See getrlimit(2) for details.

Since version 2.6.25, Linux also provides two /proc files that can be used to reserve a certain amount of CPU time to be used by non-real-time processes. Reserving some CPU time in this fashion allows some CPU time to be allocated to (say) a root shell that can be used to kill a runaway process. Both of these files specify time values in microseconds:

/proc/sys/kernel/sched_rt_period_us

This file specifies a scheduling period that is equivalent to 100% CPU bandwidth. The value in this file can range from 1 to INT_MAX, giving an operating range of 1 microsecond to around 35 minutes. The default value in this file is 1,000,000 (1 second).

/proc/sys/kernel/sched_rt_runtime_us

The value in this file specifies how much of the "period" time can be used by all real-time and deadline scheduled processes on the system. The value in this file can range from -1 to INT_MAX-1. Specifying -1 makes the runtime the same as the period; that is, no CPU time is set aside for non-real-time processes (which was the Linux behavior before kernel 2.6.25). The default value in this file is 950,000 (0.95 seconds), meaning that 5% of the CPU time is reserved for processes that don't run under a real-time or deadline scheduling policy.

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).  

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

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 are then used to run a thread with true real-time priority and a minimum worst-case scheduling latency.  

SEE ALSO

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

The Linux kernel source files Documentation/scheduler/sched-deadline.txt, Documentation/scheduler/sched-rt-group.txt, Documentation/scheduler/sched-design-CFS.txt, and Documentation/scheduler/sched-nice-design.txt

Index

NAME

DESCRIPTION

API summary

Scheduling policies

SCHED_FIFO: First in-first out scheduling

SCHED_RR: Round-robin scheduling

SCHED_DEADLINE: Sporadic task model deadline scheduling

SCHED_OTHER: Default Linux time-sharing scheduling

SCHED_BATCH: Scheduling batch processes

SCHED_IDLE: Scheduling very low priority jobs

Resetting scheduling policy for child processes

Privileges and resource limits

Limiting the CPU usage of real-time and deadline processes

Response time

Miscellaneous

NOTES

Real-time features in the mainline Linux kernel

SEE ALSO