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NAME

       sched_setscheduler,   sched_getscheduler   -  set  and  get  scheduling
       policy/parameters

SYNOPSIS

       #include <sched.h>

       int sched_setscheduler(pid_t pid, int policy,
                              const struct sched_param *param);

       int sched_getscheduler(pid_t pid);

       struct sched_param {
           ...
           int sched_priority;
           ...
       };

DESCRIPTION

       sched_setscheduler() sets both the scheduling policy and the associated
       parameters for the process whose ID is specified in pid.  If pid equals
       zero, the scheduling policy and parameters of the calling process  will
       be  set.   The  interpretation  of  the  argument  param depends on the
       selected policy.  Currently,  Linux  supports  the  following  "normal"
       (i.e., non-real-time) scheduling policies:

       SCHED_OTHER   the standard round-robin time-sharing policy;

       SCHED_BATCH   for "batch" style execution of processes; and

       SCHED_IDLE    for running very low priority background jobs.

       The  following  "real-time"  policies  are  also supported, for special
       time-critical applications that need precise control over  the  way  in
       which runnable processes are selected for execution:

       SCHED_FIFO    a first-in, first-out policy; and

       SCHED_RR      a round-robin policy.

       The semantics of each of these policies are detailed below.

       sched_getscheduler() queries the scheduling policy currently applied to
       the process identified by pid.  If pid equals zero, the policy  of  the
       calling process will be retrieved.

   Scheduling Policies
       The  scheduler  is  the  kernel  component  that decides which runnable
       process will be  executed  by  the  CPU  next.   Each  process  has  an
       associated   scheduling   policy  and  a  static  scheduling  priority,
       sched_priority;  these  are  the  settings   that   are   modified   by
       sched_setscheduler().   The  scheduler  makes  it  decisions  based  on
       knowledge of the scheduling policy and static priority of all processes
       on the system.

       For  processes  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 processes always have higher
       priority than normal processes.)  Note well: POSIX.1-2001 only requires
       an  implementation to support 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 processes for
       each possible  sched_priority  value.   In  order  to  determine  which
       process  runs  next, the scheduler looks for the nonempty list with the
       highest static priority and selects the process at  the  head  of  this
       list.

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

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

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

       *  A SCHED_FIFO process that has been preempted by another  process  of
          higher  priority  will stay at the head of the list for its priority
          and will resume  execution  as  soon  as  all  processes  of  higher
          priority are blocked again.

       *  When  a  SCHED_FIFO process becomes runnable, it will be inserted at
          the end of the list for its priority.

       *  A call to sched_setscheduler() or  sched_setparam(2)  will  put  the
          SCHED_FIFO  (or  SCHED_RR) process identified by pid at the start of
          the list if it was runnable.  As a consequence, it may  preempt  the
          currently   running   process   if   it   has   the  same  priority.
          (POSIX.1-2001 specifies that the process should go to the end of the
          list.)

       *  A process calling sched_yield(2) will be put at the end of the list.

       No  other  events  will  move  a process scheduled under the SCHED_FIFO
       policy in the  wait  list  of  runnable  processes  with  equal  static
       priority.

       A SCHED_FIFO process runs until either it is blocked by an I/O request,
       it  is  preempted  by  a  higher  priority   process,   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 process
       is  only  allowed  to  run  for  a maximum time quantum.  If a SCHED_RR
       process 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 process that has been preempted by a higher priority process
       and  subsequently  resumes execution as a running process 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_OTHER: Default Linux time-sharing scheduling
       SCHED_OTHER  can only be used at static priority 0.  SCHED_OTHER is the
       standard  Linux  time-sharing  scheduler  that  is  intended  for   all
       processes  that  do  not require the special real-time mechanisms.  The
       process 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 (set by nice(2) or  setpriority(2))
       and  increased  for  each time quantum the process is ready to run, but
       denied to run by the scheduler.  This ensures fair progress  among  all
       SCHED_OTHER processes.

   SCHED_BATCH: Scheduling batch processes
       (Since  Linux 2.6.16.)  SCHED_BATCH can only be used at static priority
       0.  This policy is similar to SCHED_OTHER  in  that  it  schedules  the
       process  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 process is CPU-intensive.  Consequently, the scheduler
       will apply a small scheduling penalty with respect to wakeup behaviour,
       so that this process 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 only be used 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
       Since  Linux 2.6.32, the SCHED_RESET_ON_FORK flag can be ORed in policy
       when calling sched_setscheduler().  As a result of including this flag,
       children  created  by  fork(2)  do  not  inherit  privileged scheduling
       policies.  This 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  SCHED_RESET_ON_FORK  flag  is  specified,  the
       following rules apply for subsequently created children:

       *  If  the  calling  process  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  SCHED_RESET_ON_FORK  flag  has been enabled, it can only be
       reset if the process has the CAP_SYS_NICE  capability.   This  flag  is
       disabled in child processes created by fork(2).

       The SCHED_RESET_ON_FORK flag is visible in the policy value returned by
       sched_getscheduler()

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

       Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a  ceiling
       on  an  unprivileged  process'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 process 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 process can also
          make these changes, as long as the effective user ID of the  process
          making  the  change  matches  the  real  or effective user ID of the
          target process.

       *  Special rules apply for the SCHED_IDLE.   In  Linux  kernels  before
          2.6.39,  an  unprivileged process 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
          process can switch to either the  SCHED_BATCH  or  the  SCHED_NORMAL
          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) processes 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.

   Response time
       A  blocked  high  priority  process  waiting  for the 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).

       Since  a  nonblocking  infinite  loop  in  a  process  scheduled  under
       SCHED_FIFO or SCHED_RR will block all  processes  with  lower  priority
       forever,  a  software  developer  should  always  keep available on the
       console a shell scheduled under  a  higher  static  priority  than  the
       tested  application.  This will allow an emergency kill of tested real-
       time applications that do not block or terminate as expected.  See also
       the description of the RLIMIT_RTTIME resource limit in getrlimit(2).

       POSIX  systems  on  which sched_setscheduler() and sched_getscheduler()
       are available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>.

RETURN VALUE

       On   success,   sched_setscheduler()   returns   zero.    On   success,
       sched_getscheduler()  returns the policy for the process (a nonnegative
       integer).  On error, -1 is returned, and errno is set appropriately.

ERRORS

       EINVAL The scheduling policy is not one  of  the  recognized  policies,
              param is NULL, or param does not make sense for the policy.

       EPERM  The calling process does not have appropriate privileges.

       ESRCH  The process whose ID is pid could not be found.

CONFORMING TO

       POSIX.1-2001  (but  see  BUGS  below).   The SCHED_BATCH and SCHED_IDLE
       policies are Linux-specific.

NOTES

       POSIX.1 does not detail the permissions that  an  unprivileged  process
       requires in order to call sched_setscheduler(), and details vary across
       systems.  For example, the Solaris 7 manual page says that the real  or
       effective user ID of the calling process must match the real user ID or
       the save set-user-ID of the target process.

       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.

   Real-time features in the mainline Linux kernel
       From kernel version 2.6.18 onward, however, Linux is gradually becoming
       equipped with real-time capabilities, most of which  are  derived  from
       the  former  realtime-preempt  patches developed by Ingo Molnar, Thomas
       Gleixner, Steven Rostedt, and others.   Until  the  patches  have  been
       completely  merged  into  the  mainline  kernel (this is expected to be
       around kernel version 2.6.30), 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 that can be selected using sched_setscheduler() are then  used
       to  run a process with true real-time priority and a minimum worst-case
       scheduling latency.

BUGS

       POSIX says that on  success,  sched_setscheduler()  should  return  the
       previous   scheduling  policy.   Linux  sched_setscheduler()  does  not
       conform to this requirement, since it always returns 0 on success.

SEE ALSO

       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_rr_get_interval(2),
       sched_setaffinity(2),         sched_setparam(2),        sched_yield(2),
       setpriority(2), 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   kernel   source  file  Documentation/scheduler/sched-rt-group.txt
       (since kernel 2.6.25).

COLOPHON

       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.