Ubuntu Manpage: sched - overview of scheduling APIs

NAME

       sched - overview of scheduling APIs

DESCRIPTION

API summary
The Linux scheduling APIs are as follows:

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
threads 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 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 SCHED_FIFO thread becomes runnable, it will be inserted at the end of the list for its
priority.

* A call to sched_setscheduler(2), sched_setparam(2), or sched_setattr(2) will put the SCHED_FIFO (or
SCHED_RR) thread identified by pid at the start of the list if it was runnable. As a consequence, it
may preempt the currently running thread if it has the same priority. (POSIX.1 specifies that the
thread should go to the end of the list.)

* 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 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 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 fulfil 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 will fail 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. 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 (set by nice(2), setpriority(2),
or sched_setattr(2)) and 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.

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
In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE) threads can set a nonzero static priority
(i.e., set a real-time scheduling policy). The only change that an unprivileged thread can make is to
set the SCHED_OTHER policy, and this can be done only if the effective user ID of the caller matches the
real or effective user ID of the target thread (i.e., the thread specified by pid) whose policy is being
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. 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)).

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 will block all threads with lower priority forever. Prior to Linux 2.6.25, the only way of
preventing a runaway real-time process from freezing the system was to run (at the console) a shell
scheduled under a higher static priority than the tested application. This allows an emergency kill of
tested real-time applications that do not block or terminate as expected.

Since Linux 2.6.25, there are other techniques for dealing with runaway real-time and deadline processes.
One of these is to use the RLIMIT_RTTIME resource limit to set a ceiling on the CPU time that a real-time
process may consume. See getrlimit(2) for 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 some 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 runtime 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.

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).

NOTES

       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.

   Real-time features in the mainline Linux kernel
       From kernel version  2.6.18  onward,  however,  Linux  is  gradually  becoming  equipped  with  real-time
       capabilities,  most  of  which  are  derived  from  the former realtime-preempt patches developed by Ingo
       Molnar, Thomas Gleixner, Steven Rostedt, and others.  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.

COLOPHON

       This  page  is  part  of  release  4.04  of  the  Linux man-pages project.  A description of the project,
       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
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