SCHED(7) Linux Programmer's Manual SCHED(7)
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 minimum priority available in a specified scheduling policy.
sched_get_priority_min(2)
Return the maximum 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 func-
tionality 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.1-2001 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 par-
ticular 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 pre-
empted 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 algo-
rithm 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.1-2001 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 rel-
ative 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 struc-
ture; see sched_setattr(2). These fields express value 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), setprior-
ity(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 behav-
ior, 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 mak-
ing 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 can-
not 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_NORMAL 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 pol-
icy 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 dead-
line 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 appli-
cations, 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 determinis-
tic 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 (this is expected to be around kernel version 2.6.30), 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 poli-
cies 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
Linux 2014-10-02 SCHED(7)