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man: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 run time 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
     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 4.16 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/.

Linux 2018-02-02 SCHED(7)

/data/webs/external/dokuwiki/data/pages/man/sched.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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