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

       sched_setscheduler,  sched_getscheduler  -  set and get scheduling pol-

       #include <&lt;sched.h>&gt;

       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 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  associ-
       ated  scheduling  policy and a static scheduling priority, sched_prior-
       ity; 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 non-empty list with  the
       highest  static  priority  and  selects the process at the head of this

       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  prior-
       ity  becomes  ready  to run, the currently running process will be pre-
       empted 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 with-
       out 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 prior-
          ity 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

       *  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 pol-
       icy 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_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  pro-
       cesses  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

   Privileges and resource limits
       In  Linux  kernels  before  2.6.12, only privileged (CAP_SYS_NICE) pro-
       cesses can set a non-zero static priority (i.e., set a real-time sched-
       uling  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 pri-
       ority 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

       * Special rules apply for the SCHED_IDLE: an unprivileged process oper-
         ating 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  pol-
       icy   and  priority.   See  getrlimit(2)  for  further  information  on

   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.

       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  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  con-
       sole  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 &lt;unistd.h&gt;.

       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.

       EINVAL The  scheduling policy is not one of the recognized policies, 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.

       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 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  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 onwards, however, Linux is gradually  becom-
       ing  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:


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

       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 process 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.

       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),   setprior-
       ity(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).

       This  page  is  part of release 3.05 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                             2008-06-27             SCHED_SETSCHEDULER(2)