man-pages/man7/sched.7

.\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\" Various pieces from the old sched_setscheduler(2) page
.\" 	Copyright (C) Tom Bjorkholm, Markus Kuhn & David A. Wheeler 1996-1999
.\" 	and Copyright (C) 2007 Carsten Emde <Carsten.Emde@osadl.org>
.\" 	and Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.TH SCHED 7 2014-04-28 "Linux" "Linux Programmer's Manual"
.SH NAME
sched \- overview of scheduling APIs
.SH DESCRIPTION
.SS API summary
The Linux scheduling APIs are as follows:
.TP
.BR sched_setscheduler (2)
Set the scheduling policy and parameters of a specified thread.
.TP
.BR sched_getscheduler (2)
Return the scheduling policy of a specified thread.
.TP
.BR sched_setparam (2)
Set the scheduling parameters of a specified thread.
.TP
.BR sched_getparam (2)
Fetch the scheduling parameters of a specified thread.
.TP
.BR sched_get_priority_max (2)
Return the minimum priority available in a specified scheduling policy.
.TP
.BR sched_get_priority_min (2)
Return the maximum priority available in a specified scheduling policy.
.TP
.BR sched_rr_get_interval (2)
Fetch the quantum used for threads that are scheduled under
the "round-robin" scheduling policy.
.TP
.BR sched_yield (2)
Cause the caller to relinquish the CPU,
so that some other thread be executed.
.TP
.BR sched_setaffinity (2)
(Linux-specific)
Set the CPU affinity of a specified thread.
.TP
.BR sched_getaffinity (2)
(Linux-specific)
Get the CPU affinity of a specified thread.
.TP
.BR 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
.BR sched_setscheduler (2)
and
.BR sched_setparam (2).
.TP
.BR 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
.BR sched_getscheduler (2)
and
.BR sched_getparam (2).
.\"
.SS 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 \fIstatic\fP
scheduling priority,
.IR sched_priority .
The scheduler makes its 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
(\fBSCHED_OTHER\fP, \fBSCHED_IDLE\fP, \fBSCHED_BATCH\fP),
\fIsched_priority\fP is not used in scheduling
decisions (it must be specified as 0).

Processes scheduled under one of the real-time policies
(\fBSCHED_FIFO\fP, \fBSCHED_RR\fP) have a
\fIsched_priority\fP 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
.BR sched_get_priority_min (2)
and
.BR 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 \fIsched_priority\fP 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.
.SS SCHED_FIFO: First in-first out scheduling
\fBSCHED_FIFO\fP can be used only with static priorities higher than
0, which means that when a \fBSCHED_FIFO\fP threads becomes runnable,
it will always immediately preempt any currently running
\fBSCHED_OTHER\fP, \fBSCHED_BATCH\fP, or \fBSCHED_IDLE\fP thread.
\fBSCHED_FIFO\fP is a simple scheduling
algorithm without time slicing.
For threads scheduled under the
\fBSCHED_FIFO\fP policy, the following rules apply:
.IP * 3
A \fBSCHED_FIFO\fP 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.
.IP *
When a \fBSCHED_FIFO\fP thread becomes runnable, it
will be inserted at the end of the list for its priority.
.IP *
A call to
.BR sched_setscheduler (2),
.BR sched_setparam (2),
or
.BR sched_setattr (2)
will put the
\fBSCHED_FIFO\fP (or \fBSCHED_RR\fP) thread identified by
\fIpid\fP 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.)
.\" In 2.2.x and 2.4.x, the thread is placed at the front of the queue
.\" In 2.0.x, the Right Thing happened: the thread went to the back -- MTK
.IP *
A thread calling
.BR sched_yield (2)
will be put at the end of the list.
.PP
No other events will move a thread
scheduled under the \fBSCHED_FIFO\fP policy in the wait list of
runnable threads with equal static priority.

A \fBSCHED_FIFO\fP
thread runs until either it is blocked by an I/O request, it is
preempted by a higher priority thread, or it calls
.BR sched_yield (2).
.SS SCHED_RR: Round-robin scheduling
\fBSCHED_RR\fP is a simple enhancement of \fBSCHED_FIFO\fP.
Everything
described above for \fBSCHED_FIFO\fP also applies to \fBSCHED_RR\fP,
except that each thread is allowed to run only for a maximum time
quantum.
If a \fBSCHED_RR\fP 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 \fBSCHED_RR\fP 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
.BR sched_rr_get_interval (2).
.\" On Linux 2.4, the length of the RR interval is influenced
.\" by the process nice value -- MTK
.\"
.SS SCHED_OTHER: Default Linux time-sharing scheduling
\fBSCHED_OTHER\fP can be used at only static priority 0.
\fBSCHED_OTHER\fP 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 \fIdynamic\fP priority that is determined only
inside this list.
The dynamic priority is based on the nice value (set by
.BR nice (2),
.BR setpriority (2),
or
.BR sched_setattr (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 \fBSCHED_OTHER\fP threads.
.\"
.SS SCHED_BATCH: Scheduling batch processes
(Since Linux 2.6.16.)
\fBSCHED_BATCH\fP can be used only at static priority 0.
This policy is similar to \fBSCHED_OTHER\fP 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.

.\" The following paragraph is drawn largely from the text that
.\" accompanied Ingo Molnar's patch for the implementation of
.\" SCHED_BATCH.
.\" commit b0a9499c3dd50d333e2aedb7e894873c58da3785
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).
.\"
.SS SCHED_IDLE: Scheduling very low priority jobs
(Since Linux 2.6.23.)
\fBSCHED_IDLE\fP 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
.B SCHED_OTHER
or
.B SCHED_BATCH
policies).
.\"
.SS Resetting scheduling policy for child processes
Each thread has a reset-on-fork scheduling flag.
When this flag is set, children created by
.BR fork (2)
do not inherit privileged scheduling policies.
The reset-on-fork flag can be set by either:
.IP * 3
ORing the
.B SCHED_RESET_ON_FORK
flag into the
.I policy
argument when calling
.BR sched_setscheduler (2)
(since Linux 2.6.32);
or
.IP *
specifying the
.B SCHED_FLAG_RESET_ON_FORK
flag in
.IR attr.sched_flags
when calling
.BR sched_setattr (2).
.PP
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
.BR sched_getscheduler (2)
and
.BR sched_getattr (2).

The reset-on-fork feature is intended for media-playback applications,
and can be used to prevent applications evading the
.BR RLIMIT_RTTIME
resource limit (see
.BR 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:
.IP * 3
If the calling thread has a scheduling policy of
.B SCHED_FIFO
or
.BR SCHED_RR ,
the policy is reset to
.BR SCHED_OTHER
in child processes.
.IP *
If the calling process has a negative nice value,
the nice value is reset to zero in child processes.
.PP
After the reset-on-fork flag has been enabled,
it can be reset only if the thread has the
.BR CAP_SYS_NICE
capability.
This flag is disabled in child processes created by
.BR fork (2).
.\"
.SS Privileges and resource limits
In Linux kernels before 2.6.12, only privileged
.RB ( 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
.B SCHED_OTHER
policy, and this can be done only if the effective user ID of the caller
matches the real or effective user ID of the target thread
(i.e., the thread specified by
.IR pid )
whose policy is being changed.

Since Linux 2.6.12, the
.B RLIMIT_RTPRIO
resource limit defines a ceiling on an unprivileged thread's
static priority for the
.B SCHED_RR
and
.B SCHED_FIFO
policies.
The rules for changing scheduling policy and priority are as follows:
.IP * 3
If an unprivileged thread has a nonzero
.B 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
.B RLIMIT_RTPRIO
soft limit.
.IP *
If the
.B 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.
.IP *
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.
.IP *
Special rules apply for the
.BR 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
.BR RLIMIT_RTPRIO
resource limit.
In Linux kernels since 2.6.39,
.\" commit c02aa73b1d18e43cfd79c2f193b225e84ca497c8
an unprivileged thread can switch to either the
.BR SCHED_BATCH
or the
.BR SCHED_NORMAL
policy so long as its nice value falls within the range permitted by its
.BR RLIMIT_NICE
resource limit (see
.BR getrlimit (2)).
.PP
Privileged
.RB ( CAP_SYS_NICE )
threads ignore the
.B RLIMIT_RTPRIO
limit; as with older kernels,
they can make arbitrary changes to scheduling policy and priority.
See
.BR getrlimit (2)
for further information on
.BR RLIMIT_RTPRIO .
.SS Response time
A blocked high priority thread waiting for 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.
.\" as described in
.\" .BR request_irq (9).
.SS Miscellaneous
Child processes inherit the scheduling policy and parameters across a
.BR fork (2).
The scheduling policy and parameters are preserved across
.BR execve (2).

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

Since a nonblocking infinite loop in a thread scheduled under
\fBSCHED_FIFO\fP or \fBSCHED_RR\fP 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
.BR RLIMIT_RTTIME
resource limit in
.BR getrlimit (2).
.SH NOTES
.PP
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.
.SS Real-time features in the mainline Linux kernel
.\" FIXME . Probably this text will need some minor tweaking
.\" by about the time of 2.6.30; ask Carsten Emde about this then.
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
.I 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:
.in +4n
.nf

patch-\fIkernelversion\fP-rt\fIpatchversion\fP
.fi
.in
.PP
and can be downloaded from
.UR http://www.kernel.org\:/pub\:/linux\:/kernel\:/projects\:/rt/
.UE .

Without the patches and prior to their full inclusion into the mainline
kernel, the kernel configuration offers only the three preemption classes
.BR CONFIG_PREEMPT_NONE ,
.BR CONFIG_PREEMPT_VOLUNTARY ,
and
.B 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
.B 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.
.SH SEE ALSO
.ad l
.nh
.BR chrt (1),
.BR getpriority (2),
.BR mlock (2),
.BR mlockall (2),
.BR munlock (2),
.BR munlockall (2),
.BR nice (2),
.BR sched_get_priority_max (2),
.BR sched_get_priority_min (2),
.BR sched_getscheduler (2),
.BR sched_getaffinity (2),
.BR sched_getparam (2),
.BR sched_rr_get_interval (2),
.BR sched_setaffinity (2),
.BR sched_setscheduler (2),
.BR sched_setparam (2),
.BR sched_yield (2),
.BR setpriority (2),
.BR pthread_getaffinity_np (3),
.BR pthread_setaffinity_np (3),
.BR sched_getcpu (3),
.BR capabilities (7),
.BR cpuset (7)
.ad
.PP
.I Programming for the real world \- POSIX.4
by Bill O. Gallmeister, O'Reilly & Associates, Inc., ISBN 1-56592-074-0.
.PP
The Linux kernel source files
.IR Documentation/scheduler/sched-deadline.txt ,
.IR Documentation/scheduler/sched-rt-group.txt ,
.IR Documentation/scheduler/sched-design-CFS.txt ,
and
.IR Documentation/scheduler/sched-nice-design.txt