bionic (7) sched.7.gz

Provided by: manpages_4.15-1_all bug

NAME

       sched - overview of CPU scheduling

DESCRIPTION

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

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

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

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

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

       sched_setscheduler(2)
              Set the scheduling policy and parameters of a specified thread.

       sched_getscheduler(2)
              Return the scheduling policy of a specified thread.

       sched_setparam(2)
              Set the scheduling parameters of a specified thread.

       sched_getparam(2)
              Fetch the scheduling parameters of a specified thread.

       sched_get_priority_max(2)
              Return the maximum priority available in a specified scheduling policy.

       sched_get_priority_min(2)
              Return the minimum priority available in a specified scheduling policy.

       sched_rr_get_interval(2)
              Fetch the quantum used for threads that are scheduled under the "round-robin" scheduling policy.

       sched_yield(2)
              Cause the caller to relinquish the CPU, so that some other thread be executed.

       sched_setaffinity(2)
              (Linux-specific) Set the CPU affinity of a specified thread.

       sched_getaffinity(2)
              (Linux-specific) Get the CPU affinity of a specified thread.

       sched_setattr(2)
              Set the scheduling policy and parameters of a specified thread.  This (Linux-specific) system call
              provides a superset of the functionality of sched_setscheduler(2) and sched_setparam(2).

       sched_getattr(2)
              Fetch  the  scheduling  policy and parameters of a specified thread.  This (Linux-specific) system
              call provides a superset of the functionality of sched_getscheduler(2) and sched_getparam(2).

   Scheduling policies
       The scheduler is the kernel component that decides which runnable thread will  be  executed  by  the  CPU
       next.   Each thread has an associated scheduling policy and a static scheduling priority, sched_priority.
       The scheduler makes its decisions based on knowledge of the scheduling policy and static priority of  all
       threads on the system.

       For threads scheduled under one of the normal scheduling policies (SCHED_OTHER, SCHED_IDLE, SCHED_BATCH),
       sched_priority is not used in scheduling decisions (it must be specified as 0).

       Processes scheduled under one of the real-time policies  (SCHED_FIFO,  SCHED_RR)  have  a  sched_priority
       value  in  the  range  1 (low) to 99 (high).  (As the numbers imply, real-time threads always have higher
       priority than normal threads.)  Note well: POSIX.1 requires an implementation to support only  a  minimum
       32  distinct  priority  levels  for  the  real-time  policies, and some systems supply just this minimum.
       Portable programs should use sched_get_priority_min(2) and sched_get_priority_max(2) to find the range of
       priorities supported for a particular policy.

       Conceptually,  the scheduler maintains a list of runnable threads for each possible sched_priority value.
       In order to determine which thread runs next, the scheduler looks for the nonempty list with the  highest
       static priority and selects the thread at the head of this list.

       A  thread's  scheduling  policy  determines where it will be inserted into the list of threads with equal
       static priority and how it will move inside this list.

       All scheduling is preemptive: if a thread with a  higher  static  priority  becomes  ready  to  run,  the
       currently  running  thread will be preempted and returned to the wait list for its static priority level.
       The scheduling policy determines the ordering only within the list of runnable threads with equal  static
       priority.

   SCHED_FIFO: First in-first out scheduling
       SCHED_FIFO  can  be  used  only  with static priorities higher than 0, which means that when a SCHED_FIFO
       threads becomes  runnable,  it  will  always  immediately  preempt  any  currently  running  SCHED_OTHER,
       SCHED_BATCH,  or  SCHED_IDLE  thread.   SCHED_FIFO is a simple scheduling algorithm without time slicing.
       For threads scheduled under the SCHED_FIFO policy, the following rules apply:

       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 priority.

          •  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 calls.

       A  sporadic task is one that has a sequence of jobs, where each job is activated at most once per period.
       Each job also has a relative deadline, before which it should finish execution, and a  computation  time,
       which is the CPU time necessary for executing the job.  The moment when a task wakes up because a new job
       has to be executed is called the arrival time (also referred to as the request  time  or  release  time).
       The  start time is the time at which a task starts its execution.  The absolute deadline is thus obtained
       by adding the relative deadline to the arrival time.

       The following diagram clarifies these terms:

           arrival/wakeup                    absolute deadline
                |    start time                    |
                |        |                         |
                v        v                         v
           -----x--------xooooooooooooooooo--------x--------x---
                         |<- comp. time ->|
                |<------- relative deadline ------>|
                |<-------------- period ------------------->|

       When setting a SCHED_DEADLINE  policy  for  a  thread  using  sched_setattr(2),  one  can  specify  three
       parameters:  Runtime,  Deadline,  and  Period.   These  parameters  do  not necessarily correspond to the
       aforementioned terms: usual practice is to set Runtime to something bigger than the  average  computation
       time  (or  worst-case  execution  time  for hard real-time tasks), Deadline to the relative deadline, and
       Period to the period of the task.  Thus, for SCHED_DEADLINE scheduling, we have:

           arrival/wakeup                    absolute deadline
                |    start time                    |
                |        |                         |
                v        v                         v
           -----x--------xooooooooooooooooo--------x--------x---
                         |<-- Runtime ------->|
                |<----------- Deadline ----------->|
                |<-------------- Period ------------------->|

       The  three  deadline-scheduling  parameters  correspond  to  the   sched_runtime,   sched_deadline,   and
       sched_period  fields  of  the sched_attr structure; see sched_setattr(2).  These fields express values in
       nanoseconds.  If sched_period is specified as 0, then it is made the same as sched_deadline.

       The kernel requires that:

           sched_runtime <= sched_deadline <= sched_period

       In addition, under the current implementation, all of the parameter values must be at least  1024  (i.e.,
       just over one microsecond, which is the resolution of the implementation), and less than 2^63.  If any of
       these checks fails, sched_setattr(2) fails with the error EINVAL.

       The CBS guarantees non-interference between tasks, by throttling threads that attempt to  over-run  their
       specified Runtime.

       To  ensure  deadline  scheduling  guarantees,  the  kernel  must  prevent  situations  where  the  set of
       SCHED_DEADLINE threads is not feasible (schedulable) within  the  given  constraints.   The  kernel  thus
       performs  an  admittance  test  when  setting  or  changing  SCHED_DEADLINE  policy and attributes.  This
       admission test calculates whether the change is feasible; if it is not, sched_setattr(2) fails  with  the
       error EBUSY.

       For example, it is required (but not necessarily sufficient) for the total utilization to be less than or
       equal to the total number of CPUs available, where, since each thread can maximally run for  Runtime  per
       Period, that thread's utilization is its Runtime divided by its Period.

       In  order to fulfill the guarantees that are made when a thread is admitted to the SCHED_DEADLINE policy,
       SCHED_DEADLINE threads are the highest priority  (user  controllable)  threads  in  the  system;  if  any
       SCHED_DEADLINE thread is runnable, it will preempt any thread scheduled under one of the other policies.

       A  call  to  fork(2)  by  a thread scheduled under the SCHED_DEADLINE policy 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.

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

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

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

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

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

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

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

   SCHED_BATCH: Scheduling batch processes
       (Since  Linux  2.6.16.)   SCHED_BATCH  can  be used only at static priority 0.  This policy is similar to
       SCHED_OTHER in that it schedules the thread according to its dynamic priority (based on the nice  value).
       The  difference  is  that  this  policy will cause the scheduler to always assume that the thread is CPU-
       intensive.  Consequently, the scheduler will apply a small scheduling  penalty  with  respect  to  wakeup
       behavior, so that this thread is mildly disfavored in scheduling decisions.

       This  policy  is useful for workloads that are noninteractive, but do not want to lower their nice value,
       and for workloads that want  a  deterministic  scheduling  policy  without  interactivity  causing  extra
       preemptions (between the workload's tasks).

   SCHED_IDLE: Scheduling very low priority jobs
       (Since  Linux  2.6.23.)   SCHED_IDLE can be used only at static priority 0; the process nice value has no
       influence for this policy.

       This policy is intended for running jobs at extremely low priority (lower even than a +19 nice value with
       the SCHED_OTHER or SCHED_BATCH policies).

   Resetting scheduling policy for child processes
       Each  thread  has a reset-on-fork scheduling flag.  When this flag is set, children created by fork(2) do
       not inherit privileged scheduling policies.  The reset-on-fork flag can be set by either:

       *  ORing the SCHED_RESET_ON_FORK flag into the policy argument when calling sched_setscheduler(2)  (since
          Linux 2.6.32); or

       *  specifying the SCHED_FLAG_RESET_ON_FORK flag in attr.sched_flags when calling sched_setattr(2).

       Note  that  the  constants used with these two APIs have different names.  The state of the reset-on-fork
       flag can analogously be retrieved using sched_getscheduler(2) and sched_getattr(2).

       The reset-on-fork feature is intended for  media-playback  applications,  and  can  be  used  to  prevent
       applications  evading  the  RLIMIT_RTTIME  resource  limit  (see getrlimit(2)) by creating multiple child
       processes.

       More precisely, if the reset-on-fork flag is set, the following  rules  apply  for  subsequently  created
       children:

       *  If  the  calling  thread  has  a  scheduling  policy of SCHED_FIFO or SCHED_RR, the policy is reset to
          SCHED_OTHER in child processes.

       *  If the calling process has a negative nice value, the nice value is reset to zero in child processes.

       After the reset-on-fork flag has been enabled, it can be reset only if the thread  has  the  CAP_SYS_NICE
       capability.  This flag is disabled in child processes created by fork(2).

   Privileges and resource limits
       In  Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE) threads can set a nonzero static priority
       (i.e., set a real-time scheduling policy).  The only change that an unprivileged thread can  make  is  to
       set  the SCHED_OTHER policy, and this can be done only if the effective user ID of the caller matches the
       real or effective user ID of the target thread (i.e., the thread specified by pid) whose policy is  being
       changed.

       A thread must be privileged (CAP_SYS_NICE) in order to set or modify a SCHED_DEADLINE policy.

       Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a ceiling on an unprivileged thread's static
       priority for the SCHED_RR and SCHED_FIFO policies.  The rules for changing scheduling policy and priority
       are as follows:

       *  If  an  unprivileged  thread has a nonzero RLIMIT_RTPRIO soft limit, then it can change its scheduling
          policy and priority, subject to the restriction that the priority cannot be set to a value higher than
          the maximum of its current priority and its RLIMIT_RTPRIO soft limit.

       *  If the RLIMIT_RTPRIO soft limit is 0, then the only permitted changes are to lower the priority, or to
          switch to a non-real-time policy.

       *  Subject to the same rules, another unprivileged thread can also make these changes,  as  long  as  the
          effective  user ID of the thread making the change matches the real or effective user ID of the target
          thread.

       *  Special rules apply for the SCHED_IDLE policy.  In Linux kernels before 2.6.39, an unprivileged thread
          operating  under  this  policy  cannot change its policy, regardless of the value of its RLIMIT_RTPRIO
          resource limit.  In Linux kernels since 2.6.39, an  unprivileged  thread  can  switch  to  either  the
          SCHED_BATCH  or  the  SCHED_OTHER policy so long as its nice value falls within the range permitted by
          its RLIMIT_NICE resource limit (see getrlimit(2)).

       Privileged (CAP_SYS_NICE) threads ignore the RLIMIT_RTPRIO limit; as with older kernels,  they  can  make
       arbitrary  changes  to  scheduling  policy  and  priority.   See  getrlimit(2) for further information on
       RLIMIT_RTPRIO.

   Limiting the CPU usage of real-time and deadline processes
       A nonblocking infinite loop in a thread scheduled  under  the  SCHED_FIFO,  SCHED_RR,  or  SCHED_DEADLINE
       policy  can  potentially  block all other threads from accessing the CPU forever.  Prior to Linux 2.6.25,
       the only way of preventing a runaway real-time process from freezing  the  system  was  to  run  (at  the
       console)  a  shell  scheduled under a higher static priority than the tested application.  This allows an
       emergency kill of tested real-time applications that do not block or terminate as expected.

       Since Linux 2.6.25, there are other techniques for dealing with runaway real-time and deadline processes.
       One of these is to use the RLIMIT_RTTIME resource limit to set a ceiling on the CPU time that a real-time
       process may consume.  See getrlimit(2) for details.

       Since version 2.6.25, Linux also provides two /proc files that can be used to reserve a certain amount of
       CPU  time to be used by non-real-time processes.  Reserving CPU time in this fashion allows some CPU time
       to be allocated to (say) a root shell that can be used to kill a runaway process.  Both  of  these  files
       specify time values in microseconds:

       /proc/sys/kernel/sched_rt_period_us
              This  file  specifies  a scheduling period that is equivalent to 100% CPU bandwidth.  The value in
              this file can range from 1 to INT_MAX, giving an operating range of 1  microsecond  to  around  35
              minutes.  The default value in this file is 1,000,000 (1 second).

       /proc/sys/kernel/sched_rt_runtime_us
              The  value  in  this file specifies how much of the "period" time can be used by all real-time and
              deadline scheduled processes on the system.   The  value  in  this  file  can  range  from  -1  to
              INT_MAX-1.   Specifying  -1  makes the runtime the same as the period; that is, no CPU time is set
              aside for non-real-time processes (which was  the  Linux  behavior  before  kernel  2.6.25).   The
              default  value in this file is 950,000 (0.95 seconds), meaning that 5% of the CPU time is reserved
              for processes that don't run under a real-time or deadline scheduling policy.

   Response time
       A blocked high priority thread waiting for I/O has a certain response time before it is scheduled  again.
       The  device  driver  writer  can  greatly reduce this response time by using a "slow interrupt" interrupt
       handler.

   Miscellaneous
       Child processes inherit the scheduling policy and parameters across a fork(2).  The scheduling policy and
       parameters are preserved across execve(2).

       Memory  locking  is  usually needed for real-time processes to avoid paging delays; this can be done with
       mlock(2) or mlockall(2).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

           $ echo 10 > /proc/self/autogroup

   Real-time features in the mainline Linux kernel
       Since kernel version 2.6.18, Linux is gradually becoming equipped with real-time  capabilities,  most  of
       which  are  derived  from  the former realtime-preempt patch set.  Until the patches have been completely
       merged into the mainline kernel, they must be installed to achieve the best real-time performance.  These
       patches are named:

           patch-kernelversion-rtpatchversion

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

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

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

NOTES

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

       Originally,  Standard  Linux  was  intended  as  a  general-purpose operating system being able to handle
       background processes, interactive applications, and less demanding real-time  applications  (applications
       that need to usually meet timing deadlines).  Although the Linux kernel 2.6 allowed for kernel preemption
       and the newly introduced  O(1)  scheduler  ensures  that  the  time  needed  to  schedule  is  fixed  and
       deterministic irrespective of the number of active tasks, true real-time computing was not possible up to
       kernel version 2.6.17.

SEE ALSO

       chrt(1), taskset(1), getpriority(2), mlock(2), mlockall(2), munlock(2), munlockall(2), nice(2),
       sched_get_priority_max(2), sched_get_priority_min(2), sched_getaffinity(2), sched_getparam(2),
       sched_getscheduler(2), sched_rr_get_interval(2), sched_setaffinity(2), sched_setparam(2),
       sched_setscheduler(2), sched_yield(2), setpriority(2), pthread_getaffinity_np(3),
       pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7), cpuset(7)

       Programming for the real world - POSIX.4 by Bill  O.  Gallmeister,  O'Reilly  &  Associates,  Inc.,  ISBN
       1-56592-074-0.

       The  Linux kernel source files Documentation/scheduler/sched-deadline.txt, Documentation/scheduler/sched-
       rt-group.txt,   Documentation/scheduler/sched-design-CFS.txt,   and   Documentation/scheduler/sched-nice-
       design.txt

COLOPHON

       This  page  is  part  of  release  4.15  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/.