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       sched - overview of CPU scheduling


       Since  Linux  2.6.23,  the default scheduler is CFS, the "Completely Fair Scheduler".  The
       CFS scheduler replaced the earlier "O(1)" scheduler.

   API summary
       Linux provides the following system calls for controlling  the  CPU  scheduling  behavior,
       policy, and priority of processes (or, more precisely, threads).

              Set a new nice value for the calling thread, and return the new nice value.

              Return  the nice value of a thread, a process group, or the set of threads owned by
              a specified user.

              Set the nice value of a thread, a process group, or the set of threads owned  by  a
              specified user.

              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  thread  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 running 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 blocked SCHED_FIFO thread becomes runnable, it will be inserted at  the  end  of
          the list for its priority.

       •  If    a    call    to   sched_setscheduler(2),   sched_setparam(2),   sched_setattr(2),
          pthread_setschedparam(3),  or  pthread_setschedprio(3)  changes  the  priority  of  the
          running  or  runnable  SCHED_FIFO  thread  identified by pid the effect on the thread's
          position in the list depends on the direction of the change to threads priority:

          (a)  If the thread's priority is raised, it is placed at the end of the  list  for  its
               new  priority.   As  a consequence, it may preempt a currently running thread with
               the same priority.

          (b)  If the thread's priority is unchanged, its position in the run list is unchanged.

          (c)  If the thread's priority is lowered, it is placed at the front of the list for its
               new priority.

          According  to  POSIX.1-2008,  changes  to  a  thread's  priority  (or policy) using any
          mechanism other than pthread_setschedprio(3) should result in the thread  being  placed
          at the end of the list for its priority.

       •  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  Linux  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 fulfill 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 fails 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 (i.e., threads under real-time policies
       always have priority over SCHED_OTHER processes).  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
       (see below) and is 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.

       In the Linux kernel source code, the SCHED_OTHER policy is actually named SCHED_NORMAL.

   The nice value
       The nice value is an attribute that can be used to influence the CPU scheduler to favor or
       disfavor a process in scheduling decisions.  It affects the scheduling of SCHED_OTHER  and
       SCHED_BATCH  (see  below)  processes.   The  nice  value  can  be  modified using nice(2),
       setpriority(2), or sched_setattr(2).

       According to POSIX.1, the nice value is a per-process attribute; that is, the threads in a
       process  should  share  a  nice  value.  However, on Linux, the nice value is a per-thread
       attribute: different threads in the same process may have different nice values.

       The range of the nice value varies across UNIX systems.  On modern Linux, the range is -20
       (high priority) to +19 (low priority).  On some other systems, the range is -20..20.  Very
       early Linux kernels (before Linux 2.0) had the range -infinity..15.

       The degree to which  the  nice  value  affects  the  relative  scheduling  of  SCHED_OTHER
       processes likewise varies across UNIX systems and across Linux kernel versions.

       With  the  advent  of  the  CFS scheduler in Linux 2.6.23, Linux adopted an algorithm that
       causes relative differences in nice values to have a much stronger effect.  In the current
       implementation,  each  unit of difference in the nice values of two processes results in a
       factor of 1.25 in the degree to which the scheduler favors the  higher  priority  process.
       This  causes  very low nice values (+19) to truly provide little CPU to a process whenever
       there is any other higher priority load on the system, and makes high  nice  values  (-20)
       deliver most of the CPU to applications that require it (e.g., some audio applications).

       On  Linux,  the  RLIMIT_NICE  resource  limit  can  be  used to define a limit to which an
       unprivileged process's nice value can be raised; see setrlimit(2) for details.

       For further details on the nice value, see the subsections on the  autogroup  feature  and
       group scheduling, below.

   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
       Before Linux 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.  Before Linux 2.6.39, an unprivileged
          thread operating under this policy cannot change its policy, regardless of the value of
          its  RLIMIT_RTPRIO  resource  limit.   Since  Linux  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 can potentially block all  other  threads  from  accessing  the  CPU
       forever.  Before 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  Linux  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 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 run time the same as the
              period; that is, no CPU time is set aside for non-real-time  processes  (which  was
              the behavior before Linux 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).

   The autogroup feature
       Since  Linux  2.6.38,  the  kernel  provides  a  feature  known as autogrouping to improve
       interactive desktop performance in the face of multiprocess, CPU-intensive workloads  such
       as  building  the  Linux  kernel with large numbers of parallel build processes (i.e., the
       make(1) -j flag).

       This feature operates in conjunction with the CFS scheduler and requires a kernel that  is
       configured  with  CONFIG_SCHED_AUTOGROUP.  On a running system, this feature is enabled or
       disabled via the file /proc/sys/kernel/sched_autogroup_enabled; a value of 0 disables  the
       feature,  while  a value of 1 enables it.  The default value in this file is 1, unless the
       kernel was booted with the noautogroup parameter.

       A new autogroup is created when a new session is created via setsid(2); this happens,  for
       example, when a new terminal window is started.  A new process created by fork(2) inherits
       its parent's autogroup membership.  Thus, all of the processes in a session are members of
       the  same autogroup.  An autogroup is automatically destroyed when the last process in the
       group terminates.

       When autogrouping is enabled, all of the members of an autogroup are placed  in  the  same
       kernel  scheduler "task group".  The CFS scheduler employs an algorithm that equalizes the
       distribution of CPU cycles across task groups.   The  benefits  of  this  for  interactive
       desktop performance can be described via the following example.

       Suppose  that  there are two autogroups competing for the same CPU (i.e., presume either a
       single CPU system or the use of taskset(1) to confine all the processes to the same CPU on
       an  SMP  system).   The  first  group contains ten CPU-bound processes from a kernel build
       started with make -j10.  The other contains a single CPU-bound process:  a  video  player.
       The  effect  of  autogrouping  is  that  the  two groups will each receive half of the CPU
       cycles.  That is, the video player will receive 50% of the CPU cycles, rather than just 9%
       of  the  cycles,  which would likely lead to degraded video playback.  The situation on an
       SMP system is more complex, but the general effect is the same: the scheduler  distributes
       CPU  cycles across task groups such that an autogroup that contains a large number of CPU-
       bound processes does not end up hogging CPU cycles at the expense of the other jobs on the

       A   process's   autogroup   (task   group)   membership   can   be  viewed  via  the  file

           $ cat /proc/1/autogroup
           /autogroup-1 nice 0

       This file can also be used to modify the CPU bandwidth allocated to an autogroup.  This is
       done  by  writing  a  number  in  the "nice" range to the file to set the autogroup's nice
       value.  The allowed range is from +19 (low priority) to  -20  (high  priority).   (Writing
       values outside of this range causes write(2) to fail with the error EINVAL.)

       The  autogroup nice setting has the same meaning as the process nice value, but applies to
       distribution of CPU cycles to the autogroup as a whole, based on the relative nice  values
       of  other  autogroups.  For a process inside an autogroup, the CPU cycles that it receives
       will be a product of the autogroup's nice value (compared to  other  autogroups)  and  the
       process's nice value (compared to other processes in the same autogroup.

       The use of the cgroups(7) CPU controller to place processes in cgroups other than the root
       CPU cgroup overrides the effect of autogrouping.

       The autogroup  feature  groups  only  processes  scheduled  under  non-real-time  policies
       (SCHED_OTHER,  SCHED_BATCH,  and SCHED_IDLE).  It does not group processes scheduled under
       real-time and deadline policies.  Those processes are scheduled  according  to  the  rules
       described earlier.

   The nice value and group scheduling
       When  scheduling  non-real-time  processes  (i.e.,  those scheduled under the SCHED_OTHER,
       SCHED_BATCH, and SCHED_IDLE policies), the CFS scheduler  employs  a  technique  known  as
       "group  scheduling",  if the kernel was configured with the CONFIG_FAIR_GROUP_SCHED option
       (which is typical).

       Under group scheduling, threads are scheduled  in  "task  groups".   Task  groups  have  a
       hierarchical relationship, rooted under the initial task group on the system, known as the
       "root task group".  Task groups are formed in the following circumstances:

       •  All of the threads in a CPU cgroup form a task group.  The parent of this task group is
          the task group of the corresponding parent cgroup.

       •  If  autogrouping is enabled, then all of the threads that are (implicitly) placed in an
          autogroup (i.e., the same session, as created by setsid(2)) form a  task  group.   Each
          new  autogroup is thus a separate task group.  The root task group is the parent of all
          such autogroups.

       •  If autogrouping is enabled, then the root task group consists of all processes  in  the
          root CPU cgroup that were not otherwise implicitly placed into a new autogroup.

       •  If  autogrouping is disabled, then the root task group consists of all processes in the
          root CPU cgroup.

       •  If  group  scheduling  was  disabled  (i.e.,  the   kernel   was   configured   without
          CONFIG_FAIR_GROUP_SCHED), then all of the processes on the system are notionally placed
          in a single task group.

       Under group scheduling, a thread's nice value has an effect for scheduling decisions  only
       relative  to  other threads in the same task group.  This has some surprising consequences
       in terms of the traditional semantics of the nice value on UNIX systems.   In  particular,
       if autogrouping is enabled (which is the default in various distributions), then employing
       setpriority(2) or nice(1) on a process has an effect only for scheduling relative to other
       processes executed in the same session (typically: the same terminal window).

       Conversely,  for  two  processes  that  are  (for example) the sole CPU-bound processes in
       different sessions (e.g., different terminal windows, each  of  whose  jobs  are  tied  to
       different  autogroups), modifying the nice value of the process in one of the sessions has
       no effect in terms of the scheduler's decisions relative  to  the  process  in  the  other
       session.   A  possibly useful workaround here is to use a command such as the following to
       modify the autogroup nice value for all of the processes in a terminal session:

           $ echo 10 > /proc/self/autogroup

   Real-time features in the mainline Linux kernel
       Since Linux 2.6.18, Linux is gradually becoming equipped with real-time capabilities, most
       of  which  are derived from the former realtime-preempt patch set.  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.


       The cgroups(7) CPU controller can be used to  limit  the  CPU  consumption  of  groups  of

       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 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 Linux 2.6.17.


       chcpu(1), chrt(1), lscpu(1), ps(1), taskset(1), top(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_getschedparam(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