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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:
arrival/wakeup absolute deadline
| start time |
| | |
v v v
-----x--------xooooooooooooooooo--------x--------x---
|<- comp. time ->|
|<------- relative deadline ------>|
|<-------------- period ------------------->|
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:
arrival/wakeup absolute deadline
| start time |
| | |
v v v
-----x--------xooooooooooooooooo--------x--------x---
|<-- Runtime ------->|
|<----------- Deadline ----------->|
|<-------------- Period ------------------->|
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.
SEE ALSO
chrt(1), taskset(1), getpriority(2), mlock(2), mlockall(2), munlock(2), munlockall(2),
nice(2), sched_get_priority_max(2), sched_get_priority_min(2), sched_getscheduler(2),
sched_getaffinity(2), sched_getparam(2), sched_rr_get_interval(2), sched_setaffinity(2),
sched_setscheduler(2), sched_setparam(2), sched_yield(2), setpriority(2),
pthread_getaffinity_np(3), pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7),
cpuset(7)
Programming for the real world - POSIX.4 by Bill O. Gallmeister, O'Reilly & Associates,
Inc., ISBN 1-56592-074-0.
The Linux kernel source files Documentation/scheduler/sched-deadline.txt,
Documentation/scheduler/sched-rt-group.txt, Documentation/scheduler/sched-design-CFS.txt,
and Documentation/scheduler/sched-nice-design.txt
COLOPHON
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