NAME | DESCRIPTION | NOTES | SEE ALSO | COLOPHON

SCHED(7)                  Linux Programmer's Manual                 SCHED(7)

NAME         top

       sched - overview of scheduling APIs

DESCRIPTION         top

   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         top

       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 main‐
       line kernel, the kernel configuration offers only the three preemp‐
       tion classes CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY, and CON‐
       FIG_PREEMPT_DESKTOP which respectively provide no, some, and consid‐
       erable reduction of the worst-case scheduling latency.

       With the patches applied or after their full inclusion into the main‐
       line 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         top

       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_getaffinity(2), sched_getparam(2),
       sched_getscheduler(2), sched_rr_get_interval(2),
       sched_setaffinity(2), sched_setparam(2), sched_setscheduler(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         top

       This page is part of release 4.08 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
       https://www.kernel.org/doc/man-pages/.

Linux                            2016-10-08                         SCHED(7)