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SCHED(7)                   Linux Programmer's ManualSCHED(7)NAME       sched - overview of CPU schedulingDESCRIPTION       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 ofsched_setscheduler(2) andsched_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  ofsched_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 usesched_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 in-       side this list.       All scheduling is preemptive: if a thread with a higher static priority       becomes ready to run, the currently running thread  will  be  preempted       and  returned  to  the  wait  list  for its static priority level.  The       scheduling policy determines the  ordering  only  within  the  list  of       runnable threads with equal static priority.   SCHED_FIFO: First in-first out scheduling       SCHED_FIFO can be used only with static priorities higher than 0, which       means that when a SCHED_FIFO thread becomes runnable,  it  will  always       immediately  preempt any currently running SCHED_OTHER, SCHED_BATCH, or       SCHED_IDLE thread.  SCHED_FIFO is a simple scheduling algorithm without       time  slicing.   For threads scheduled under the SCHED_FIFO policy, the       following rules apply:       1) A running SCHED_FIFO thread  that  has  been  preempted  by  another          thread  of higher priority will stay at the head of the list for its          priority and will resume execution as soon as all threads of  higher          priority are blocked again.       2) When  a  blocked  SCHED_FIFO thread becomes runnable, it will be in-          serted at the end of the list for its priority.       3) If a call  tosched_setscheduler(2),sched_setparam(2),  sched_se-tattr(2),pthread_setschedparam(3),   orpthread_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:          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 thanpthread_setschedprio(3) should          result in the thread being placed at the end of  the  list  for  its          priority.       4) A thread callingsched_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  callssched_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 usingsched_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 at-       tributes,  one  must  use  the  Linux-specificsched_setattr(2)   andsched_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 ar-       rival 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 abso-       lute 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_se-tattr(2), one can specify three parameters: Runtime, Deadline, and  Pe-       riod.   These parameters do not necessarily correspond to the aforemen-       tioned 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;  seesched_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 ad-       mittance test when setting or changing SCHED_DEADLINE  policy  and  at-       tributes.   This admission test calculates whether the change is feasi-       ble; 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 ad-       mitted 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 tofork(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 callssched_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  in-       tended for all threads that do not require the special real-time mecha-       nisms.       The thread to run is chosen from the static priority 0 list based on  a       dynamic priority that is determined only inside this list.  The dynamic       priority is based on the nice value (see below) and  is  increased  for       each  time quantum the thread is ready to run, but denied to run by the       scheduler.  This ensures fair progress among all SCHED_OTHER threads.       In the Linux kernel source code, the  SCHED_OTHER  policy  is  actually       named SCHED_NORMAL.   The nice value       The  nice  value  is an attribute that can be used to influence the CPU       scheduler to favor or disfavor a process in scheduling  decisions.   It       affects  the scheduling of SCHED_OTHER and SCHED_BATCH (see below) pro-       cesses.  The nice value can be modified usingnice(2),setpriority(2),       orsched_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 byfork(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          callingsched_setscheduler(2) (since Linux 2.6.32); or       *  specifying  the  SCHED_FLAG_RESET_ON_FORK  flag  in attr.sched_flags          when callingsched_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 usingsched_getscheduler(2) andsched_getattr(2).       The reset-on-fork feature is intended for media-playback  applications,       and  can  be used to prevent applications evading the RLIMIT_RTTIME re-       source limit (seegetrlimit(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 byfork(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 be-          fore 2.6.39, an unprivileged thread operating under this policy can-          not 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  pol-          icy  so  long  as its nice value falls within the range permitted by          its RLIMIT_NICE resource limit (seegetrlimit(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.   Seegetrlimit(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.  Seegetrlimit(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 pe-              riod; 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 afork(2).  The scheduling policy and parameters are preserved across ex-ecve(2).       Memory  locking is usually needed for real-time processes to avoid pag-       ing delays; this can be done withmlock(2) ormlockall(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., themake(1) -j flag).       This  feature  operates  in  conjunction with the CFS scheduler and re-       quires 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 viasetsid(2);       this  happens,  for  example, when a new terminal window is started.  A       new process created byfork(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 oftaskset(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 causeswrite(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 thecgroups(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  (im-          plicitly) placed in an autogroup (i.e., the same session, as created          bysetsid(2)) form a task group.  Each new autogroup is thus a sepa-          rate  task group.  The root task group is the parent of all such au-          togroups.       *  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 se-       mantics of the nice value on UNIX systems.   In  particular,  if  auto-       grouping  is  enabled  (which is the default in various distributions),       then employingsetpriority(2) ornice(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 re-       duction 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 be-       comes available.  If this is selected, Linux is transformed into a reg-       ular  real-time  operating system.  The FIFO and RR scheduling policies       are then used to run a thread with true real-time priority and a  mini-       mum worst-case scheduling latency.NOTES       Thecgroups(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 introducedO(1)  scheduler       ensures that the time needed to schedule is fixed and deterministic ir-       respective of the number of active tasks, true real-time computing  was       not possible up to kernel version 2.6.17.SEE ALSOchcpu(1),chrt(1),lscpu(1),ps(1),taskset(1),top(1),getpriority(2),mlock(2),mlockall(2),munlock(2),munlockall(2),nice(2),sched_get_priority_max(2),sched_get_priority_min(2),sched_getaffinity(2),sched_getparam(2),sched_getscheduler(2),sched_rr_get_interval(2),sched_setaffinity(2),sched_setparam(2),sched_setscheduler(2),sched_yield(2),setpriority(2),pthread_getaffinity_np(3),pthread_getschedparam(3),pthread_setaffinity_np(3),sched_getcpu(3),capabilities(7),cpuset(7)       Programming for the real  world  -  POSIX.4  by  Bill  O.  Gallmeister,       O'Reilly & Associates, Inc., ISBN 1-56592-074-0.       The    Linux   kernel   source   files   Documentation/scheduler/sched-       deadline.txt,               Documentation/scheduler/sched-rt-group.txt,       Documentation/scheduler/sched-design-CFS.txt,                       and       Documentation/scheduler/sched-nice-design.txtCOLOPHON       This page is part of release 5.05 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                             2019-08-02SCHED(7)