sched_setscheduler

       icy/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 thread whose ID is specified in pid.  If pid equals
       zero, the scheduling policy and parameters of the calling  thread  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 threads 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 thread identified by pid.  If pid equals zero, the  policy  of  the
       calling thread will be retrieved.

   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;
       these are the settings that are modified by sched_setscheduler().   The
       scheduler  makes it decisions based on knowledge of the scheduling pol-
       icy 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
       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:

       *  A 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  SCHED_FIFO  thread 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)  thread identified by pid at the start of
          the list if it was runnable.  As a consequence, it may  preempt  the
          currently running thread if it has the same priority.  (POSIX.1-2001
          specifies that the thread should go to the end of the list.)

       *  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  pol-
       icy in the wait list of runnable threads with equal static priority.

       A  SCHED_FIFO thread runs until either it is blocked by an I/O request,
       it  is  preempted  by  a  higher   priority   thread,   or   it   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
       SCHED_OTHER threads.

   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 behaviour,
       so that this thread is mildly disfavored in scheduling decisions.

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

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

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

   Resetting scheduling policy for child processes
       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 poli-
       cies.  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  fol-
       lowing 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  SCHED_RESET_ON_FORK  flag has been enabled, it can be reset
       only if the thread has the CAP_SYS_NICE capability.  This flag is  dis-
       abled 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)  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  of  sched_setscheduler()  matches  the  real  or
          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.   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_NORMAL pol-
          icy  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 pol-
       icy  and  priority.   See  getrlimit(2)  for  further  information   on
       RLIMIT_RTPRIO.

   Response time
       A  blocked  high  priority  thread  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  pag-
       ing delays; this can be done with mlock(2) or mlockall(2).

       Since   a  nonblocking  infinite  loop  in  a  thread  scheduled  under
       SCHED_FIFO or SCHED_RR will block all threads with lower priority  for-
       ever,  a software developer should always keep available on the console
       a shell scheduled under a higher static priority than the tested appli-
       cation.  This will allow an emergency kill of tested real-time applica-
       tions 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 thread (a nonnegative
       integer).  On error, -1 is returned, and errno is set appropriately.

NOTES
       POSIX.1 does not detail the permissions  that  an  unprivileged  thread
       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 caller must match the real user ID or the save
       set-user-ID of the target.

       The scheduling policy and parameters are in fact per-thread  attributes
       on Linux.  The value returned from a call to gettid(2) can be passed in
       the argument pid.  Specifying pid as 0 will operate  on  the  attribute
       for  the  calling thread, and passing the value returned from a call to
       getpid(2) will operate on the attribute for  the  main  thread  of  the
       thread  group.   (If  you  are  using  the  POSIX threads API, then use
       pthread_setschedparam(3),         pthread_getschedparam(3),         and
       pthread_setschedprio(3), instead of the sched_*(2) system calls.)

       Originally,  Standard Linux was intended as a general-purpose operating
       system being able to handle background processes, interactive  applica-
       tions,  and  less  demanding  real-time applications (applications that
       need to usually meet timing deadlines).  Although the Linux kernel  2.6
       allowed  for  kernel preemption and the newly 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 com-
       pletely 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_PRE-
       EMPT_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 main-
       line  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  poli-
       cies  that  can be selected using sched_setscheduler() are then used to
       run a thread with true real-time  priority  and  a  minimum  worst-case
       scheduling latency.

BUGS
       O'Reilly & Associates, Inc., ISBN 1-56592-074-0.

       The Linux kernel source file Documentation/scheduler/sched-rt-group.txt

COLOPHON
       This page is part of release 3.54 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/.



Linux                             2013-09-17             SCHED_SETSCHEDULER(2)
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