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Linux 2.6 - man page for sched_setscheduler (linux section 2)

SCHED_SETSCHEDULER(2)		    Linux Programmer's Manual		    SCHED_SETSCHEDULER(2)

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
       thread whose ID is specified in pid.  If pid equals zero, the scheduling policy and param-
       eters 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 applica-
       tions 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 identi-
       fied 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_setsched-
       uler().	 The scheduler makes it 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.1-2001
       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 priori-
       ties 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 run-
       ning  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 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 execu-
       tion as a running thread will complete the unexpired portion of its round-robin time quan-
       tum.  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  be  used  at  only static priority 0.	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 (set by nice(2) or setpriority(2)) and 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.

   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 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
       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 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 disabled	in  child  processes  created  by
       fork(2).

       The  SCHED_RESET_ON_FORK  flag  is visible in the policy value returned by sched_getsched-
       uler()

   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 effec-
       tive user ID of the target thread (i.e., the thread specified  by  pid)	whose  policy  is
       being changed.

       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.  In Linux kernels before 2.6.39, an unprivi-
	  leged 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 unprivi-
	  leged thread 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) 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.

   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 sched-
       uling 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 thread scheduled under SCHED_FIFO or SCHED_RR  will
       block  all  threads  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 thread (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 thread does not have appropriate privileges.

       ESRCH  The thread 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-spe-
       cific.

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 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_PRE-
       EMPT_VOLUNTARY, and CONFIG_PREEMPT_DESKTOP which respectively provide no, some,	and  con-
       siderable reduction of the worst-case scheduling latency.

       With the patches applied or after their full inclusion into the mainline kernel, the addi-
       tional 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  thread
       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
       chrt(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_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 Linux kernel source file Documentation/scheduler/sched-rt-group.txt

COLOPHON
       This  page  is  part of release 3.55 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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