sched — overview of scheduling APIs
The Linux scheduling APIs are as follows:
Set the scheduling policy and parameters of a specified thread.
Return the scheduling policy of a specified thread.
Set the scheduling parameters of a specified thread.
Fetch the scheduling parameters of a specified thread.
Return the maximum priority available in a specified scheduling policy.
Return the minimum priority available in a specified scheduling policy.
Fetch the quantum used for threads that are scheduled under the "round-robin" scheduling policy.
Cause the caller to relinquish the CPU, so that some other thread be executed.
(Linux-specific) Set the CPU affinity of a specified thread.
(Linux-specific) Get the CPU affinity of a specified thread.
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,
The scheduler makes its decisions based on knowledge of the
scheduling policy and static priority of all threads on the
For threads scheduled under one of the normal scheduling
sched_priority is not used
in scheduling decisions (it must be specified as 0).
Processes scheduled under one of the real-time policies
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 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)
to find the range of priorities supported for a particular
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
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 can be used
only with static priorities higher than 0, which means that
becomes runnable, it will always immediately preempt any
SCHED_FIFO is a simple scheduling
algorithm without time slicing. For threads scheduled under
SCHED_FIFO policy, the
following rules apply:
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.
thread becomes runnable, it will be inserted at the
end of the list for its priority.
A call to sched_setscheduler(2),
will put the
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 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.
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_RR is a simple
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
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
Since version 3.14, Linux provides a deadline scheduling
policy is currently implemented using GEDF (Global Earliest
Deadline First) in conjunction with CBS (Constant Bandwidth
Server). To set and fetch this policy and associated
attributes, one must use the Linux-specific sched_setattr(2) and
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:
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
arrival/wakeup absolute deadline | start time | | | | v v v -----x--------xooooooooooooooooo--------x--------x--- |<-- Runtime ------->| |<----------- Deadline ----------->| |<-------------- Period ------------------->|
The three deadline-scheduling parameters correspond to
fields of the
sched_attr structure; see
fields express values in nanoseconds. If
sched_period is specified
as 0, then it is made the same as
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 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).
that calls sched_yield(2) will yield
the current job and wait for a new period to begin.
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), setpriority(2), or
increased for each time quantum the thread is ready to run,
but denied to run by the scheduler. This ensures fair
progress among all
(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 behavior,
so that this thread is mildly disfavored in scheduling
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).
(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 is intended for running jobs at extremely
low priority (lower even than a +19 nice value with the
Each thread has a reset-on-fork scheduling flag. When this flag is set, children created by fork(2) do not inherit privileged scheduling policies. The reset-on-fork flag can be set by either:
The reset-on-fork feature is intended for media-playback
applications, and can be used to prevent applications
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_RR, the policy is
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).
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 matches the
real or effective user ID of the target thread (i.e., the
thread specified by
pid) whose policy is being
A thread must be privileged (
CAP_SYS_NICE) in order to set or modify a
Since Linux 2.6.12, the
RLIMIT_RTPRIO resource limit defines a
ceiling on an unprivileged thread's static priority for the
SCHED_FIFO policies. The rules for
changing scheduling policy and priority are as follows:
If an unprivileged thread has a nonzero
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.
soft limit is 0, then the only permitted changes are
to lower the priority, or to switch to a
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_OTHER policy so long as its
nice value falls within the range permitted by its
limit (see getrlimit(2)).
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
A nonblocking infinite loop in a thread scheduled under
SCHED_DEADLINE policy will block all
threads with lower priority forever. Prior to Linux 2.6.25,
the only way of preventing a runaway real-time process from
freezing the system was to run (at the console) a shell
scheduled under a higher static priority than the tested
application. This allows an emergency kill of tested
real-time applications that do not block or terminate as
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
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:
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).
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
−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.
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.
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.
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
developed by Ingo Molnar, Thomas Gleixner, Steven Rostedt,
and others. Until the patches have been completely merged
into the mainline kernel, they must be installed to achieve
the best real-time performance. These patches are
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
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
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
chrt(1), taskset(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 files
This page is part of release 4.07 of the Linux
man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
Copyright (C) 2014 Michael Kerrisk <mtk.manpagesgmail.com>
and Copyright (C) 2014 Peter Zijlstra <peterzinfradead.org>
and Copyright (C) 2014 Juri Lelli <juri.lelligmail.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.Emdeosadl.org>
and Copyright (C) 2008 Michael Kerrisk <mtk.manpagesgmail.com>
This is free documentation; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2 of
the License, or (at your option) any later version.
The GNU General Public License's references to "object code"
and "executables" are to be interpreted as the output of any
document formatting or typesetting system, including
intermediate and printed output.
This manual is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public
License along with this manual; if not, see
Worth looking at: http://rt.wiki.kernel.org/index.php