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)"
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
- 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
- (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_getparam(2).
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
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 (
is not used in scheduling decisions (it must be
specified as 0).
Processes scheduled under one of the real-time policies ( SCHED_FIFO
) 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
(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
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
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
, or SCHED_IDLE
is a simple
scheduling algorithm without time slicing. For threads scheduled under the
policy, the following rules apply:
- 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.
- When a blocked SCHED_FIFO thread 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_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:
- 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.
- If the thread's priority is unchanged, its position in the run list is
- 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 policy)
using any mechanism other than pthread_setschedprio(3) should
result in the thread being placed at the end of the list for its
- A thread calling sched_yield(2) will be put at the end of the
No other events will move a thread scheduled under the SCHED_FIFO
in the wait list of runnable threads with equal static priority.
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
is a simple enhancement of SCHED_FIFO
described above for SCHED_FIFO
also applies to SCHED_RR
that each thread is allowed to run only for a maximum time quantum. If a
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.
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
Since version 3.14, Linux provides a deadline scheduling policy
). 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 attributes, one must use
the Linux-specific sched_setattr
(2) and sched_getattr
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
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
referred to as the request time or release time). The start time
time at which a task starts its execution. The absolute deadline
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
|<- comp. time ->|
|<------- relative deadline ------>|
|<-------------- period ------------------->|
When setting a SCHED_DEADLINE
policy for a thread using
(2), one can specify three parameters: Runtime
, and 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
arrival/wakeup absolute deadline
| start time |
| | |
v v v
|<-- Runtime ------->|
|<----------- Deadline ----------->|
|<-------------- Period ------------------->|
The three deadline-scheduling parameters correspond to the sched_runtime
, and sched_period
fields of the sched_attr
structure; see sched_setattr
(2). These fields express values in
nanoseconds. If sched_period
is specified as 0, then it is made the
same as sched_deadline
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,
(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,
(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 admitted to
threads are the
highest priority (user controllable) threads in the system; if any
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).
thread that calls sched_yield
(2) will yield the
current job and wait for a new period to begin.
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
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
In the Linux kernel source code, the SCHED_OTHER
policy is actually named
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
(see below) processes.
The nice value can be modified using nice
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
The degree to which the nice value affects the relative scheduling of
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
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
(2) for details.
For further details on the nice value, see the subsections on the autogroup
feature and group scheduling, below.
(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 deterministic scheduling
policy without interactivity causing extra preemptions (between the workload's
(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
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
(2) and sched_getattr
The reset-on-fork feature is intended for media-playback applications, and can
be used to prevent applications evading the RLIMIT_RTTIME
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
- 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
In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE
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
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 changed.
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
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
- 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 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
RLIMIT_NICE resource limit (see getrlimit(2)).
) threads ignore the RLIMIT_RTPRIO
as with older kernels, they can make arbitrary changes to scheduling policy
and priority. See getrlimit
(2) for further information on
A nonblocking infinite loop in a thread scheduled under the SCHED_FIFO
, 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 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
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 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'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.
Child processes inherit the scheduling policy and parameters across a
(2). The scheduling policy and parameters are preserved across
Memory locking is usually needed for real-time processes to avoid paging delays;
this can be done with mlock
(2) or mlockall
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
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 file
; 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
A new autogroup is created when a new session is created via setsid
this happens, for example, when a new terminal window is started. A new
process created by fork
(2) inherits its parent'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
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'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's nice value. The allowed range is from +19 (low
priority) to -20 (high priority). (Writing values outside of this range causes
(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 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
, and SCHED_IDLE
does not group processes scheduled under real-time and deadline policies.
Those processes are scheduled according to the rules described earlier.
When scheduling non-real-time processes (i.e., those scheduled under the
, and SCHED_IDLE
CFS scheduler employs a technique known as "group scheduling", if
the kernel was configured with the CONFIG_FAIR_GROUP_SCHED
(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 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's nice value has an effect for scheduling
decisions only relative to other threads in the same task group
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
(1) on a process has an effect only for scheduling relative to
other processes executed in the same session (typically: the same terminal
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 following to modify the
autogroup nice value for all
of the processes in a terminal session:
$ echo 10 > /proc/self/autogroup
Since kernel version 2.6.18, Linux is gradually becoming equipped with real-time
capabilities, most of which are derived from the former
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
Without the patches and prior to their full inclusion into the mainline kernel,
the kernel configuration offers only the three preemption classes
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
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.
(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.
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-deadline.txt