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
       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.

       In the Linux kernel source code, the SCHED_OTHER policy is actually
       named SCHED_NORMAL.

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

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

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

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

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

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

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

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

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

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

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

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

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

       *  specifying the SCHED_FLAG_RESET_ON_FORK flag in attr.sched_flags
          when calling 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 run time the same as the
              period; that is, no CPU time is set aside for non-real-time
              processes (which was the Linux behavior before kernel 2.6.25).
              The default value in this file is 950,000 (0.95 seconds),
              meaning that 5% of the CPU time is reserved for processes that
              don't run under a real-time or deadline scheduling policy.

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

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

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

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

COLOPHON
       This page is part of release 5.03 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                             2019-08-02                          SCHED(7)