Section (7) sched
sched — overview of CPU scheduling
Since Linux 2.6.23, the default scheduler is CFS, the Completely Fair Scheduler. The CFS scheduler replaced the earlier O(1) scheduler.
Linux provides the following system calls for controlling the CPU scheduling behavior, policy, and priority of processes (or, more precisely, threads).
Set a new nice value for the calling thread, and return the new nice value.
Return the nice value of a thread, a process group, or the set of threads owned by a specified user.
Set the nice value of a thread, a process group, or the set of threads owned by a specified user.
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_zsingle_quotesz_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
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:
SCHED_FIFOthread 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 blocked
SCHED_FIFOthread becomes runnable, it will be inserted at the end of the list for its priority.
If a call to sched_setscheduler(2), sched_setparam(2), sched_setattr(2), pthread_setschedparam(3), or pthread_setschedprio(3) changes the priority of the running or runnable
SCHED_FIFOthread identified by
pidthe effect on the thread_zsingle_quotesz_s position in the list depends on the direction of the change to threads priority:
If the thread_zsingle_quotesz_s priority is raised, it is placed at the end of the list for its new priority. As a consequence, it may preempt a currently running thread with the same priority.
If the thread_zsingle_quotesz_s priority is unchanged, its position in the run list is unchanged.
If the thread_zsingle_quotesz_s priority is lowered, it is placed at the front of the list for its new priority.
According to POSIX.1-2008, changes to a thread_zsingle_quotesz_s priority (or policy) using any mechanism other than pthread_setschedprio(3) should result in the thread being placed at the end of the list for its priority.
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: Round-robin scheduling
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
SCHED_DEADLINE: Sporadic task model deadline scheduling
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_zsingle_quotesz_s utilization is its Runtime divided by its Period.
In order to fulfill 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 fails 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: Default Linux time-sharing scheduling
SCHED_OTHER can be used at
only static priority 0 (i.e., threads under real-time
policies always have priority over
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 (see below) and is increased for
each time quantum the thread is ready to run, but denied to
run by the scheduler. This ensures fair progress among all
In the Linux kernel source code, the
SCHED_OTHER policy is actually named
The nice value
The nice value is an attribute that can be used to
influence the CPU scheduler to favor or disfavor a process
in scheduling decisions. It affects the scheduling of
SCHED_BATCH (see below) processes. The
nice value can be modified using nice(2), setpriority(2), or
According to POSIX.1, the nice value is a per-process attribute; that is, the threads in a process should share a nice value. However, on Linux, the nice value is a per-thread attribute: different threads in the same process may have different nice values.
The range of the nice value varies across UNIX systems. On modern Linux, the range is −20 (high priority) to +19 (low priority). On some other systems, the range is −20..20. Very early Linux kernels (Before Linux 2.0) had the range −infinity..15.
The degree to which the nice value affects the relative
processes likewise varies across UNIX systems and across
Linux kernel versions.
With the advent of the CFS scheduler in kernel 2.6.23, Linux adopted an algorithm that causes relative differences in nice values to have a much stronger effect. In the current implementation, each unit of difference in the nice values of two processes results in a factor of 1.25 in the degree to which the scheduler favors the higher priority process. This causes very low nice values (+19) to truly provide little CPU to a process whenever there is any other higher priority load on the system, and makes high nice values (−20) deliver most of the CPU to applications that require it (e.g., some audio applications).
On Linux, the
resource limit can be used to define a limit to which an
unprivileged process_zsingle_quotesz_s nice value can be raised; see
For further details on the nice value, see the subsections on the autogroup feature and group scheduling, below.
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 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_zsingle_quotesz_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 is intended for running jobs at extremely
low priority (lower even than a +19 nice value with the
Resetting scheduling policy for child processes
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 reset to
SCHED_OTHERin 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
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_zsingle_quotesz_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
RLIMIT_RTPRIOsoft 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_RTPRIOsoft 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_IDLEpolicy. 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_RTPRIOresource limit. In Linux kernels since 2.6.39, an unprivileged thread can switch to either the
SCHED_OTHERpolicy so long as its nice value falls within the range permitted by its
RLIMIT_NICEresource 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
Limiting the CPU usage of real-time and deadline processes
A nonblocking infinite loop in a thread scheduled under
SCHED_DEADLINE policy can potentially
block all other threads from accessing the CPU 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 expected.
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 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
INT_MAX−1. Specifying −1 makes the run time 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_zsingle_quotesz_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.
The autogroup feature
Since Linux 2.6.38, the kernel provides a feature known
as autogrouping to improve interactive desktop performance
in the face of multiprocess, CPU-intensive workloads such
as building the Linux kernel with large numbers of parallel
build processes (i.e., the make(1)
This feature operates in conjunction with the CFS
scheduler and requires a kernel that is configured with
CONFIG_SCHED_AUTOGROUP. On a
running system, this feature is enabled or disabled via the
a value of 0 disables the feature, while a value of 1
enables it. The default value in this file is 1, unless the
kernel was booted with the
A new autogroup is created when a new session is created via setsid(2); this happens, for example, when a new terminal window is started. A new process created by fork(2) inherits its parent_zsingle_quotesz_s autogroup membership. Thus, all of the processes in a session are members of the same autogroup. An autogroup is automatically destroyed when the last process in the group terminates.
When autogrouping is enabled, all of the members of an autogroup are placed in the same kernel scheduler task group. The CFS scheduler employs an algorithm that equalizes the distribution of CPU cycles across task groups. The benefits of this for interactive desktop performance can be described via the following example.
Suppose that there are two autogroups competing for the same CPU (i.e., presume either a single CPU system or the use of taskset(1) to confine all the processes to the same CPU on an SMP system). The first group contains ten CPU-bound processes from a kernel build started with make −j10. The other contains a single CPU-bound process: a video player. The effect of autogrouping is that the two groups will each receive half of the CPU cycles. That is, the video player will receive 50% of the CPU cycles, rather than just 9% of the cycles, which would likely lead to degraded video playback. The situation on an SMP system is more complex, but the general effect is the same: the scheduler distributes CPU cycles across task groups such that an autogroup that contains a large number of CPU-bound processes does not end up hogging CPU cycles at the expense of the other jobs on the system.
A process_zsingle_quotesz_s autogroup (task group) membership can be
viewed via the file
$ cat /proc/1/autogroup /autogroup-1 nice 0
This file can also be used to modify the CPU bandwidth allocated to an autogroup. This is done by writing a number in the nice range to the file to set the autogroup_zsingle_quotesz_s nice value. The allowed range is from +19 (low priority) to −20 (high priority). (Writing values outside of this range causes write(2) to fail with the error EINVAL.)
The autogroup nice setting has the same meaning as the process nice value, but applies to distribution of CPU cycles to the autogroup as a whole, based on the relative nice values of other autogroups. For a process inside an autogroup, the CPU cycles that it receives will be a product of the autogroup_zsingle_quotesz_s nice value (compared to other autogroups) and the process_zsingle_quotesz_s nice value (compared to other processes in the same autogroup.
The use of the cgroups(7) CPU controller to place processes in cgroups other than the root CPU cgroup overrides the effect of autogrouping.
The autogroup feature groups only processes scheduled
under non-real-time policies (
SCHED_IDLE). It does not group processes
scheduled under real-time and deadline policies. Those
processes are scheduled according to the rules described
The nice value and group scheduling
When scheduling non-real-time processes (i.e., those
scheduled under the
SCHED_IDLE policies), the CFS scheduler
employs a technique known as group scheduling, if the
kernel was configured with the
CONFIG_FAIR_GROUP_SCHED option (which is
Under group scheduling, threads are scheduled in task groups. Task groups have a hierarchical relationship, rooted under the initial task group on the system, known as the root task group. Task groups are formed in the following circumstances:
All of the threads in a CPU cgroup form a task group. The parent of this task group is the task group of the corresponding parent cgroup.
If autogrouping is enabled, then all of the threads that are (implicitly) placed in an autogroup (i.e., the same session, as created by setsid(2)) form a task group. Each new autogroup is thus a separate task group. The root task group is the parent of all such autogroups.
If autogrouping is enabled, then the root task group consists of all processes in the root CPU cgroup that were not otherwise implicitly placed into a new autogroup.
If autogrouping is disabled, then the root task group consists of all processes in the root CPU cgroup.
If group scheduling was disabled (i.e., the kernel was configured without
CONFIG_FAIR_GROUP_SCHED), then all of the processes on the system are notionally placed in a single task group.
Under group scheduling, a thread_zsingle_quotesz_s nice value has an effect for scheduling decisions only relative to other threads in the same task group. This has some surprising consequences in terms of the traditional semantics of the nice value on UNIX systems. In particular, if autogrouping is enabled (which is the default in various distributions), then employing setpriority(2) or nice(1) on a process has an effect only for scheduling relative to other processes executed in the same session (typically: the same terminal window).
Conversely, for two processes that are (for example) the
sole CPU-bound processes in different sessions (e.g.,
different terminal windows, each of whose jobs are tied to
different autogroups), modifying
the nice value of the process in one of the
sessions has no
effect in terms of the scheduler_zsingle_quotesz_s decisions
relative to the process in the other session. A possibly
useful workaround here is to use a command such as the
following to modify the autogroup nice value for
all of the processes in a
$ echo 10 > /proc/self/autogroup
Real-time features in the mainline Linux kernel
Since kernel version 2.6.18, Linux is gradually becoming
equipped with real-time capabilities, most of which are
derived from the former
realtime-preempt patch set.
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 named:
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
The cgroups(7) CPU controller can be used to limit the CPU consumption of groups of processes.
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.
chcpu(1), chrt(1), lscpu(1), ps(1), taskset(1), top(1), getpriority(2), mlock(2), mlockall(2), munlock(2), munlockall(2), nice(2), sched_get_priority_max(2), sched_get_priority_min(2), sched_getaffinity(2), sched_getparam(2), sched_getscheduler(2), sched_rr_get_interval(2), sched_setaffinity(2), sched_setparam(2), sched_setscheduler(2), sched_yield(2), setpriority(2), pthread_getaffinity_np(3), pthread_getschedparam(3), pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7), cpuset(7)
Programming for the real world − POSIX.4 by Bill O. Gallmeister, O_zsingle_quotesz_Reilly & Associates, Inc., ISBN 1-56592-074-0.
The Linux kernel source files
This page is part of release 5.04 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
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the License, or (at your option) any later version.
The GNU General Public License_zsingle_quotesz_s references to object code
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This manual is distributed in the hope that it will be useful,
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Worth looking at: http://rt.wiki.kernel.org/index.php