sched

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

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 pre-
       cisely, 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_get-
              param(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 imple-
       mentation to support only a minimum 32 distinct priority levels for the
       real-time  policies, and some systems supply just this minimum.  Porta-
       ble programs should use sched_get_priority_min(2) and  sched_get_prior-
       ity_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:

       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_setsched-
          prio(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:

          o  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.

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

          o  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  pol-
          icy)  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 pol-
       icy 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  dead-
       line,  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_run-
       time,  sched_deadline, and sched_period fields of the sched_attr struc-
       ture; see sched_setattr(2).  These fields express  values  in  nanosec-
       onds.   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 situ-
       ations where the set of SCHED_DEADLINE threads is not feasible (schedu-
       lable) 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  fea-
       sible; 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  cur-
       rent 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) pro-
       cesses.  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 dif-
       ference 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  lit-
       tle  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  setr-
       limit(2) for details.

       For further details on the nice value, see the subsections on the auto-
       group 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 determin-
       istic 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 tar-
       get 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 pri-
       ority 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 pol-
       icy  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 microsec-
       onds:

       /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 sec-
              ond).

       /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 pro-
              cesses (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 pag-
       ing 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  member-
       ship.   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  per-
       formance 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 dis-
       tributes CPU cycles across task groups such that an autogroup that con-
       tains 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 sched-
       uler 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 cre-
          ated 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 sched-
       uling 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  auto-
       grouping  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 fol-
       lowing 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 real-
       time-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_PRE-
       EMPT_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  main-
       line   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 poli-
       cies 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 applica-
       tions, 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 4.15 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/.

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