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       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 maximum priority available in a specified scheduling policy.

              Return the minimum 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-
              specific)   system   call   provides   a   superset   of   the   functionality   of
              sched_setscheduler(2) and sched_setparam(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

       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
       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
                         |<-- 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

       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
       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

       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

       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
       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).


       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, they must be  installed  to  achieve
       the best real-time performance.  These patches are named:


       and can be downloaded from ⟨⟩.

       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.


       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


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