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NAME

       sched - overview of CPU scheduling

DESCRIPTION

       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).

       nice(2)
              Set a new nice value for the calling thread, and return the new nice value.

       getpriority(2)
              Return  the nice value of a thread, a process group, or the set of threads owned by
              a specified user.

       setpriority(2)
              Set the nice value of a thread, a process group, or the set of threads owned  by  a
              specified user.

       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  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 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 (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.

   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 kernel 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
          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 can potentially block all  other  threads  from  accessing  the  CPU
       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 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).

   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
       system.

       A   process's   autogroup   (task   group)   membership   can   be  viewed  via  the  file
       /proc/[pid]/autogroup:

           $ 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  kernel  version  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:

           patch-kernelversion-rtpatchversion

       and can be downloaded from ⟨http://www.kernel.org/pub/linux/kernel/projects/rt/⟩.

       Without the patches and prior to their full inclusion into the mainline kernel, the kernel
       configuration   offers   only   the   three   preemption   classes    CONFIG_PREEMPT_NONE,
       CONFIG_PREEMPT_VOLUNTARY,  and CONFIG_PREEMPT_DESKTOP which respectively provide no, some,
       and considerable reduction of the worst-case scheduling latency.

       With the patches applied or after their full  inclusion  into  the  mainline  kernel,  the
       additional  configuration  item CONFIG_PREEMPT_RT becomes available.  If this is selected,
       Linux is transformed  into  a  regular  real-time  operating  system.   The  FIFO  and  RR
       scheduling  policies  are  then  used  to  run a thread with true real-time priority and a
       minimum worst-case scheduling latency.

NOTES

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

       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.

SEE ALSO

       chrt(1), taskset(1), getpriority(2), mlock(2), mlockall(2), munlock(2), munlockall(2),
       nice(2), sched_get_priority_max(2), sched_get_priority_min(2), sched_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

       This  page  is  part of release 4.13 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/.