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       sched_setscheduler,  sched_getscheduler	-  set and get scheduling pol‐

       #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;

       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
       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 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 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_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_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 prior‐
       ity  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 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

   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

   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
       effective  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 pri‐
       ority 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

       *  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

       *  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

       *  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

   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.

       Child processes inherit the scheduling policy and parameters  across  a
       fork(2).	  The  scheduling  policy  and parameters are preserved across

       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>.

       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.

       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.

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

       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:


       and  can	 be  downloaded	 from  ⟨

       Without the patches and prior to their full inclusion into the mainline
       kernel, the kernel  configuration  offers  only	the  three  preemption
       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.

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

       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

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2013-09-17		 SCHED_SETSCHEDULER(2)

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