SCHED(7) Linux Programmer's Manual SCHED(7)
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 pre-
Set a new nice value for the calling thread, and return the new
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
Return the minimum priority available in a specified scheduling
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.
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).
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_get-
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.
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
(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 requires an imple-
mentation to support only a minimum 32 distinct priority levels for the
real-time policies, and some systems supply just this minimum. Porta-
ble programs should use sched_get_priority_min(2) and sched_get_prior-
ity_max(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 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 in-
side 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 thread 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:
1) A running 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.
2) When a blocked SCHED_FIFO thread becomes runnable, it will be in-
serted at the end of the list for its priority.
3) If a call to sched_setscheduler(2), sched_setparam(2), sched_se-
tattr(2), pthread_setschedparam(3), or pthread_setschedprio(3)
changes the priority of the running or runnable SCHED_FIFO thread
identified by pid the effect on the thread's position in the list
depends on the direction of the change to threads priority:
o If the thread'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.
o If the thread's priority is unchanged, its position in the run
list is unchanged.
o If the thread'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's priority (or pol-
icy) using any mechanism other than pthread_setschedprio(3) should
result in the thread being placed at the end of the list for its
4) 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 pol-
icy 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_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_DEADLINE: Sporadic task model deadline scheduling
Since version 3.14, Linux provides a deadline scheduling policy
(SCHED_DEADLINE). This 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 at-
tributes, one must use the Linux-specific sched_setattr(2) and
sched_getattr(2) system calls.
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 dead-
line, 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 ar-
rival 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 abso-
lute deadline is thus obtained by adding the relative deadline to the
The following diagram clarifies these terms:
arrival/wakeup absolute deadline
| start time |
| | |
v v v
|<- comp. time ->|
|<------- relative deadline ------>|
|<-------------- period ------------------->|
When setting a SCHED_DEADLINE policy for a thread using sched_se-
tattr(2), one can specify three parameters: Runtime, Deadline, and Pe-
riod. These parameters do not necessarily correspond to the aforemen-
tioned 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
|<-- Runtime ------->|
|<----------- Deadline ----------->|
|<-------------- Period ------------------->|
The three deadline-scheduling parameters correspond to the sched_run-
time, sched_deadline, and sched_period fields of the sched_attr struc-
ture; see sched_setattr(2). These fields express values in nanosec-
onds. 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 situ-
ations where the set of SCHED_DEADLINE threads is not feasible (schedu-
lable) within the given constraints. The kernel thus performs an ad-
mittance test when setting or changing SCHED_DEADLINE policy and at-
tributes. This admission test calculates whether the change is feasi-
ble; 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 fulfill the guarantees that are made when a thread is ad-
mitted 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
fails 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 cur-
rent 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 processes).
SCHED_OTHER is the standard Linux time-sharing scheduler that is in-
tended for all threads that do not require the special real-time mecha-
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 SCHED_OTHER threads.
In the Linux kernel source code, the SCHED_OTHER policy is actually
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_OTHER and SCHED_BATCH (see below) pro-
cesses. The nice value can be modified using nice(2), setpriority(2),
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 scheduling of
SCHED_OTHER 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 dif-
ference 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 lit-
tle 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 RLIMIT_NICE resource limit can be used to define a limit
to which an unprivileged process's nice value can be raised; see setr-
limit(2) for details.
For further details on the nice value, see the subsections on the auto-
group 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 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 determin-
istic 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
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:
* ORing the SCHED_RESET_ON_FORK flag into the policy argument when
calling sched_setscheduler(2) (since Linux 2.6.32); or
* 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 re-
source 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
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 tar-
get 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_RR and
SCHED_FIFO policies. The rules for changing scheduling policy and pri-
ority 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
* 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
* 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
* Special rules apply for the SCHED_IDLE policy. In Linux kernels be-
fore 2.6.39, an unprivileged thread operating under this policy can-
not 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 pol-
icy 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 pol-
icy and priority. See getrlimit(2) for further information on
Limiting the CPU usage of real-time and deadline processes
A nonblocking infinite loop in a thread scheduled under the SCHED_FIFO,
SCHED_RR, or 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
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 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 microsec-
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 sec-
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 pe-
riod; that is, no CPU time is set aside for non-real-time pro-
cesses (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
Child processes inherit the scheduling policy and parameters across a
fork(2). The scheduling policy and parameters are preserved across ex-
Memory locking is usually needed for real-time processes to avoid pag-
ing delays; this can be done with mlock(2) or mlockall(2).
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) -j flag).
This feature operates in conjunction with the CFS scheduler and re-
quires a kernel that is configured with CONFIG_SCHED_AUTOGROUP. On a
running system, this feature is enabled or disabled via the file
/proc/sys/kernel/sched_autogroup_enabled; 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 noautogroup parameter.
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's autogroup member-
ship. 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 per-
formance 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 dis-
tributes CPU cycles across task groups such that an autogroup that con-
tains 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's autogroup (task group) membership can be viewed via the
$ 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'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's nice value (compared to other autogroups)
and the process's nice value (compared to other processes in the same
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_OTHER, SCHED_BATCH, and SCHED_IDLE). It does not
group processes scheduled under real-time and deadline policies. Those
processes are scheduled according to the rules described earlier.
The nice value and group scheduling
When scheduling non-real-time processes (i.e., those scheduled under
the SCHED_OTHER, SCHED_BATCH, and SCHED_IDLE policies), the CFS sched-
uler employs a technique known as "group scheduling", if the kernel was
configured with the CONFIG_FAIR_GROUP_SCHED option (which is typical).
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
* If autogrouping is enabled, then all of the threads that are (im-
plicitly) placed in an autogroup (i.e., the same session, as created
by setsid(2)) form a task group. Each new autogroup is thus a sepa-
rate task group. The root task group is the parent of all such au-
* 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's nice value has an effect for sched-
uling decisions only relative to other threads in the same task group.
This has some surprising consequences in terms of the traditional se-
mantics of the nice value on UNIX systems. In particular, if auto-
grouping 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's decisions relative to the process in the other session.
A possibly useful workaround here is to use a command such as the fol-
lowing 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 real-
time-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
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_PRE-
EMPT_DESKTOP which respectively provide no, some, and considerable re-
duction of the worst-case scheduling latency.
With the patches applied or after their full inclusion into the main-
line kernel, the additional configuration item CONFIG_PREEMPT_RT be-
comes available. If this is selected, Linux is transformed into a reg-
ular real-time operating system. The FIFO and RR scheduling policies
are then used to run a thread with true real-time priority and a mini-
mum worst-case scheduling latency.
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 applica-
tions, 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 ir-
respective 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_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_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 Documentation/scheduler/sched-
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