# sched

## 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** 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:
    
           •  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.
    
           •  When a blocked **SCHED_FIFO** thread becomes runnable, it will be
              inserted at the end of the list for its priority.
    
           •  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 the thread's priority:
    
              (a)  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.
    
              (b)  If the thread's priority is unchanged, its position in the
                   run list is unchanged.
    
              (c)  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.
    
           •  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 Linux 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 Linux 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**
           Before Linux 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.  Before Linux
              2.6.39, an unprivileged thread operating under this policy
              cannot change its policy, regardless of the value of its
              **RLIMIT_RTPRIO** resource limit.  Since Linux 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.  Before
           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 Linux 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 behavior before
                  Linux 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".  When
           disabled, the group creation happens as above, and autogroup
           membership is still visible in _/proc_ , but the autogroups are not
           used.  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 Linux 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_ -rt _patchversion_
    
           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 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 Linux 2.6.17.
    


* * *