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SCHED

Linux Programmer's Manual (7)
2014-04-28
 

NAME

sched - overview of scheduling APIs  

DESCRIPTION

 

API summary

The Linux scheduling APIs are as follows:
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_getinterval(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) Set the CPU affinity of a specified thread.
sched_setattr(2)
(Linux-specific) A generaized API for setting the scheduling policy and parameters of a specified thread.
sched_getattr(2)
(Linux-specific) A generaized API for fetching the scheduling policy and parameters of a specified thread.
 

Scheduling policies

The scheduler is the kernel component that decides which runnable thread will be executed by the CPU next. Each thread has an associated scheduling policy and a static scheduling priority, sched_priority; these are the settings that are modified by sched_setscheduler(). The scheduler makes it decisions based on knowledge of the scheduling policy and static priority of all threads on the system.

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

SCHED_FIFO can be used only with static priorities higher than 0, which means that when a SCHED_FIFO threads becomes runnable, it will always immediately preempt any currently running SCHED_OTHER, SCHED_BATCH, or SCHED_IDLE thread. SCHED_FIFO is a simple scheduling algorithm without time slicing. For threads scheduled under the SCHED_FIFO policy, the following rules apply:
*
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.
*
When a SCHED_FIFO thread 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) 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_RR is a simple enhancement of SCHED_FIFO. Everything described above for SCHED_FIFO also applies to SCHED_RR, except that each thread is allowed to run only for a maximum time quantum. If a SCHED_RR thread has been running for a time period equal to or longer than the time quantum, it will be put at the end of the list for its priority. A SCHED_RR thread that has been preempted by a higher priority thread and subsequently resumes execution as a running thread will complete the unexpired portion of its round-robin time quantum. The length of the time quantum can be retrieved using sched_rr_get_interval(2).  

SCHED_OTHER: Default Linux time-sharing scheduling

SCHED_OTHER can be used at only static priority 0. SCHED_OTHER is the standard Linux time-sharing scheduler that is intended for all threads that do not require the special real-time mechanisms. The thread to run is chosen from the static priority 0 list based on a dynamic priority that is determined only inside this list. The dynamic priority is based on the nice value (set by nice(2) or setpriority(2)) and increased for each time quantum the thread is ready to run, but denied to run by the scheduler. This ensures fair progress among all SCHED_OTHER threads.  

SCHED_BATCH: Scheduling batch processes

(Since Linux 2.6.16.) SCHED_BATCH can be used only at static priority 0. This policy is similar to SCHED_OTHER in that it schedules the thread according to its dynamic priority (based on the nice value). The difference is that this policy will cause the scheduler to always assume that the thread is CPU-intensive. Consequently, the scheduler will apply a small scheduling penalty with respect to wakeup behaviour, so that this thread is mildly disfavored in scheduling decisions.

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

(Since Linux 2.6.23.) SCHED_IDLE can be used only at static priority 0; the process nice value has no influence for this policy.

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

Since Linux 2.6.32, the SCHED_RESET_ON_FORK flag can be ORed in policy when calling sched_setscheduler(). As a result of including this flag, children created by fork(2) do not inherit privileged scheduling policies. This 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 SCHED_RESET_ON_FORK flag is specified, 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 SCHED_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).

The SCHED_RESET_ON_FORK flag is visible in the policy value returned by sched_getscheduler()  

Privileges and resource limits

In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE) threads can set a nonzero static priority (i.e., set a real-time scheduling policy). The only change that an unprivileged thread can make is to set the SCHED_OTHER policy, and this can be done only if the effective user ID of the caller of sched_setscheduler() matches the real or effective user ID of the target thread (i.e., the thread specified by pid) whose policy is being changed.

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:

*
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.
*
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. 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.  

Response time

A blocked high priority thread waiting for the I/O has a certain response time before it is scheduled again. The device driver writer can greatly reduce this response time by using a "slow interrupt" interrupt handler.  

Miscellaneous

Child processes inherit the scheduling policy and parameters across a fork(2). The scheduling policy and parameters are preserved across execve(2).

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 nonblocking infinite loop in a thread scheduled under SCHED_FIFO or SCHED_RR will block all threads 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

On success, sched_setscheduler() returns zero. On success, sched_getscheduler() returns the policy for the thread (a nonnegative integer). On error, -1 is returned, and errno is set appropriately.  

ERRORS

EINVAL
The scheduling policy is not one of the recognized policies, param is NULL, or param does not make sense for the policy.
EPERM
The calling thread does not have appropriate privileges.
ESRCH
The thread whose ID is pid could not be found.
 

CONFORMING TO

POSIX.1-2001 (but see BUGS below). The SCHED_BATCH and SCHED_IDLE policies are Linux-specific.  

NOTES

POSIX.1 does not detail the permissions that an unprivileged thread requires in order to call sched_setscheduler(), and details vary across systems. For example, the Solaris 7 manual page says that the real or effective user ID of the caller must match the real user ID or the save set-user-ID of the target.

The scheduling policy and parameters are in fact per-thread attributes on Linux. The value returned from a call to gettid(2) can be passed in the argument pid. Specifying pid as 0 will operate on the attribute for the calling thread, and passing the value returned from a call to getpid(2) will operate on the attribute for the main thread of the thread group. (If you are using the POSIX threads API, then use pthread_setschedparam(3), pthread_getschedparam(3), and pthread_setschedprio(3), instead of the sched_*(2) system calls.)

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

From kernel version 2.6.18 onward, however, Linux is gradually becoming equipped with real-time capabilities, most of which are derived from the former realtime-preempt patches developed by Ingo Molnar, Thomas Gleixner, Steven Rostedt, and others. Until the patches have been completely merged into the mainline kernel (this is expected to be around kernel version 2.6.30), they must be installed to achieve the best real-time performance. These patches are named:

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 that can be selected using sched_setscheduler() are then used to run a thread with true real-time priority and a minimum worst-case scheduling latency.  

BUGS

POSIX says that on success,  

SEE ALSO

chrt(1), getpriority(2), mlock(2), mlockall(2), munlock(2), munlockall(2), nice(2), sched_get_priority_max(2), sched_get_priority_min(2), sched_getscheduler(2), sched_getaffinity(2), sched_getparam(2), sched_rr_get_interval(2), sched_setaffinity(2), sched_setscheduler(2), sched_setparam(2), sched_yield(2), setpriority(2), pthread_getaffinity_np(3), pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7), cpuset(7)

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

The Linux kernel source file Documentation/scheduler/sched-rt-group.txt  

COLOPHON

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Index

NAME
DESCRIPTION
API summary
Scheduling policies
SCHED_FIFO: First in-first out scheduling
SCHED_RR: Round-robin 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
Response time
Miscellaneous
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
Real-time features in the mainline Linux kernel
BUGS
SEE ALSO
COLOPHON

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Time: 21:43:05 GMT, July 12, 2014