Ubuntu Manpage: sched - overview of CPU scheduling

NAME

       sched - overview of CPU scheduling

DESCRIPTION

Since Linux 2.6.23, the default scheduler is CFS, the "Completely Fair Scheduler". The CFS scheduler
replaced the earlier "O(1)" scheduler.

API summary
Linux provides the following system calls for controlling the CPU scheduling behavior, policy, and
priority of processes (or, more precisely, threads).

nice(2)
Set a new nice value for the calling thread, and return the new nice value.

getpriority(2)
Return the nice value of a thread, a process group, or the set of threads owned by a specified
user.

setpriority(2)
Set the nice value of a thread, a process group, or the set of threads owned by a specified user.

sched_setscheduler(2)
Set the scheduling policy and parameters of a specified thread.

sched_getscheduler(2)
Return the scheduling policy of a specified thread.

sched_setparam(2)
Set the scheduling parameters of a specified thread.

sched_getparam(2)
Fetch the scheduling parameters of a specified thread.

sched_get_priority_max(2)
Return the maximum priority available in a specified scheduling policy.

sched_get_priority_min(2)
Return the minimum priority available in a specified scheduling policy.

sched_rr_get_interval(2)
Fetch the quantum used for threads that are scheduled under the "round-robin" scheduling policy.

sched_yield(2)
Cause the caller to relinquish the CPU, so that some other thread be executed.

sched_setaffinity(2)
(Linux-specific) Set the CPU affinity of a specified thread.

sched_getaffinity(2)
(Linux-specific) Get the CPU affinity of a specified thread.

sched_setattr(2)
Set the scheduling policy and parameters of a specified thread. This (Linux-specific) system call
provides a superset of the functionality of sched_setscheduler(2) and sched_setparam(2).

sched_getattr(2)
Fetch the scheduling policy and parameters of a specified thread. This (Linux-specific) system
call provides a superset of the functionality of sched_getscheduler(2) and sched_getparam(2).

Scheduling policies
The scheduler is the kernel component that decides which runnable thread will be executed by the CPU
next. Each thread has an associated scheduling policy and a 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 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) 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 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 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 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:

• 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 the
thread's priority:

(a) 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.

(b) If the thread's priority is unchanged, its position in the run list is unchanged.

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
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.

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_yield(2).

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 Linux 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 attributes, 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 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:

When setting a SCHED_DEADLINE policy for a thread using sched_setattr(2), one can specify three
parameters: Runtime, Deadline, 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 scheduling, we have:

The three deadline-scheduling parameters correspond to the sched_runtime, sched_deadline, 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, 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 fulfill the guarantees that are made when a thread is admitted 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 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 processes). 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 SCHED_OTHER threads.

In the Linux kernel source code, the SCHED_OTHER policy is actually named SCHED_NORMAL.

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)
processes. The nice value can be modified using nice(2), setpriority(2), or sched_setattr(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 Linux 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 RLIMIT_NICE resource limit can be used to define a limit to which an unprivileged process's
nice value can be raised; see setrlimit(2) for details.

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 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 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 policies).

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 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_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
Before Linux 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 changed.

A thread must be privileged (CAP_SYS_NICE) in order to set or modify a SCHED_DEADLINE policy.

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 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 limit.

• 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 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_IDLE policy. Before Linux 2.6.39, an unprivileged thread operating
under this policy cannot change its policy, regardless of the value of its RLIMIT_RTPRIO resource
limit. Since Linux 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)).

Privileged (CAP_SYS_NICE) 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 information on
RLIMIT_RTPRIO.

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. Before 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 details.

Since Linux 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:

/proc/sys/kernel/sched_rt_period_us
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).

/proc/sys/kernel/sched_rt_runtime_us
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 behavior before Linux 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.

Response time
A blocked high priority thread waiting for I/O has a certain response time before it is scheduled again.
The device driver writer can greatly reduce this response time by using a "slow interrupt" interrupt
handler.

Miscellaneous
Child processes inherit the scheduling policy and parameters across a fork(2). The scheduling policy and
parameters are preserved across execve(2).

Memory locking is usually needed for real-time processes to avoid paging 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 requires 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
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's autogroup (task group) membership can be viewed via the file /proc/pid/autogroup:

$ 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 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_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 scheduler 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 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. 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'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

Real-time features in the mainline Linux kernel
Since Linux 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:

patch-kernelversion-rtpatchversion

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 CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY, and
CONFIG_PREEMPT_DESKTOP which respectively provide no, some, and considerable reduction of the worst-case
scheduling latency.

With the patches applied or after their full inclusion into the mainline kernel, the additional
configuration item CONFIG_PREEMPT_RT becomes available. If this is selected, Linux is transformed into a
regular real-time operating system. The FIFO and RR scheduling policies are then used to run a thread
with true real-time priority and a minimum worst-case scheduling latency.

NOTES

       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 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 Linux 2.6.17.

NAME

DESCRIPTION

NOTES

SEE ALSO