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 threads 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