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

       sched - overview of scheduling APIs

   API summary
       The Linux scheduling APIs are as follows:

              Set the scheduling policy and parameters of a specified thread.

              Return the scheduling policy of a specified thread.

              Set the scheduling parameters of a specified thread.

              Fetch the scheduling parameters of a specified thread.

              Return the minimum priority available in a specified scheduling policy.

              Return the maximum priority available in a specified scheduling policy.

              Fetch  the  quantum  used  for  threads  that are scheduled under the "round-robin"
              scheduling policy.

              Cause the caller to relinquish the CPU, so that some other thread be executed.

              (Linux-specific) Set the CPU affinity of a specified thread.

              (Linux-specific) Get the CPU affinity of a specified thread.

              Set the scheduling policy and parameters of a specified thread.   This  (Linux-spe‐
              cific)  system  call  provides  a  superset of the functionality of sched_setsched‐
              uler(2) and sched_setparam(2).

              Fetch the scheduling policy and parameters of a specified thread.  This (Linux-spe‐
              cific)  system  call  provides  a  superset of the functionality of sched_getsched‐
              uler(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  priori‐
       ties 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 run‐
       ning 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.1-2001 specifies that the thread should go to  the  end  of  the

       *  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 execu‐
       tion as a running thread will complete the unexpired portion of its round-robin time quan‐
       tum.  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

       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
                         |<- 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 cor‐
       respond  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
                         |<-- 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 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  implementa‐
       tion),  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

       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

       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 creat‐
       ing multiple child processes.

       More precisely, if the reset-on-fork flag is set, the following  rules  apply  for  subse‐
       quently 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 pol‐

       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_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 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

       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:

              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

              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.

       Child processes inherit the scheduling policy and parameters across a fork(2).  The sched‐
       uling 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).

       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

   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:


       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_PRE‐
       EMPT_VOLUNTARY,  and  CONFIG_PREEMPT_DESKTOP which respectively provide no, some, and con‐
       siderable reduction of the worst-case scheduling latency.

       With the patches applied or after their full inclusion into the mainline kernel, the addi‐
       tional 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.

       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),

       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

       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-10-02                                   SCHED(7)

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