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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  parameter 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 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 non-empty 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  non-interactive,  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).

   Privileges and resource limits
       In  Linux  kernels  before  2.6.12,  only   privileged   (CAP_SYS_NICE)
       processes  can  set  a  non-zero 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 non-zero 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:  an unprivileged process
         operating under this policy cannot change its policy,  regardless  of
         the value of its RLIMIT_RTPRIO resource limit.

       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  non-blocking  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 non-negative
       integer).  On error, -1 is returned, and errno is set appropriately.

ERRORS

       EINVAL The  scheduling policy is not one of the recognized policies, or
              the parameter 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  onwards,  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  and  others.   Until  the  patches have been
       completely merged into the mainline kernel  (this  is  expected  to  be
       around  kernel  version  2.6.24  or  2.6.25), they must be installed to
       achieve the best real-time performance.  These patches are named:

           patch-kernelversion-rtpatchversion

       and can  be  downloaded  from  http://people.redhat.com/mingo/realtime-
       preempt/.

       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 3.01 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.