[SCHED_DEADLINE man pages 2/2] sched(7) SCHED_DEADLINE

[Michael Kerrisk (man-pages)]

Hello Peter et al.

Here is the section of the sched(7) page that describes SCHED_DEADLINE,
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Cheers,

Michael

   SCHED_DEADLINE: Sporadic task model deadline scheduling
       Since version 3.14, Linux provides a deadline scheduling polâ
       icy (SCHED_DEADLINE).  This policy is  currently  implemented
       using  GEDF  (Global  Earliest Deadline First) in conjunction
       with CBS (Constant Bandwidth Server).  To set and fetch  this
       policy and associated attributes, one must use the Linux-speâ
       cific sched_setattr(2) and sched_getattr(2) system calls.

       A sporadic task is one that has a  sequence  of  jobs,  where
       each job is activated at most once per period.  Each job also
       has a relative deadline, before which it should finish execuâ
       tion, and a computation time, which is the CPU time necessary
       for executing the job.  The  moment  when  a  task  wakes  up
       because  a  new  job has to be executed is called the arrival
       time (also referred to as the request time or release  time).
       The  start time is the time at which a task starts its execuâ
       tion.  The absolute deadline is thus obtained by  adding  the
       relative deadline to the arrival time.

       The following diagram clarifies these terms:

           arrival/wakeup                   absolute deadline
                |    start time                   |
                |        |                        |
                v        v                        v
           -----x--------xooooooooooooooooo-------x--------x---
                         |<- comp. time ->|
                |<------- relative deadline ----->|
                |<-------------- period ------------------>|

       When  setting  a  SCHED_DEADLINE  policy  for  a thread using
       sched_setattr(2), one can specify three parameters:  Runtime,
       Deadline,  and  Period.   These parameters do not necessarily
       correspond to the aforementioned terms: usual practice is  to
       set  Runtime to something bigger than the average computation
       time (or worst-case execution time for hard real-time tasks),
       Deadline  to  the relative deadline, and Period to the period
       of the task.  Thus, for SCHED_DEADLINE scheduling, we have:

           arrival/wakeup                   absolute deadline
                |    start time                   |
                |        |                        |
                v        v                        v
           -----x--------xooooooooooooooooo-------x--------x---
                         |<-- Runtime --->|
                |<----------- Deadline ---------->|
                |<-------------- Period ------------------>|

       The three deadline-scheduling parameters  correspond  to  the
       sched_runtime, sched_deadline, and sched_period fields of the
       sched_attr structure;  see  sched_setattr(2).   These  fields
       express  value  in nanoseconds.  If sched_period is specified
       as 0, then it is made the same as sched_deadline.

       The kernel requires that:

           sched_runtime <= sched_deadline <= sched_period

       In addition, under the current  implementation,  all  of  the
       parameter  values  must be at least 1024 (i.e., just over one
       microsecond, which is the resolution of the  implementation).
       If any of these checks fails, sched_setattr(2) fails with the
       error EINVAL.

       The CBS guarantees non-interference between tasks, by  throtâ
       tling  threads  that attempt to over-run their specified Runâ
       time.

       To ensure deadline scheduling  guarantees,  the  kernel  must
       prevent situations where the set of SCHED_DEADLINE threads is
       not feasible (schedulable) within the given constraints.  The
       kernel  thus  performs  an  admittance  test  when setting or
       changing SCHED_DEADLINE policy and attributes.   This  admisâ
       sion test calculates whether the change is feasible; if it is
       not sched_setattr(2) fails with the error EBUSY.

       For example, it is required (but not necessarily  sufficient)
       for  the  total  utilization  to be less than or equal to the
       total number of CPUs available, where, since each thread  can
       maximally  run for Runtime per Period, that thread's utilizaâ
       tion is its Runtime divided by its Period.

       In order to fulfil the guarantees that are made when a thread
       is  admitted  to  the  SCHED_DEADLINE  policy, SCHED_DEADLINE
       threads are the highest priority (user controllable)  threads
       in  the  system; if any SCHED_DEADLINE thread is runnable, it
       will preempt any thread scheduled  under  one  of  the  other
       policies.

       A call to fork(2) by a thread scheduled under the SCHED_DEADâ
       LINE policy will fail  with  the  error  EAGAIN,  unless  the
       thread has its reset-on-fork flag set (see below).

       A  SCHED_DEADLINE thread that calls sched_yield(2) will yield
       the current job and wait for a new period to begin.

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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