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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:

       1) 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.

       2) When a blocked SCHED_FIFO thread becomes runnable, it will be inserted at  the  end  of
          the list for its priority.

       3) 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:

          •  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

          •  If the thread's priority is unchanged, its position in the run list is unchanged.

          •  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.

       4) 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 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 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

       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  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

       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:

              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 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).

   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  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:


       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  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


       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 Documentation/scheduler/sched-nice-design.txt


       This page is part of release 5.10 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