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[20170109] Documenting NetBSD's scheduler tweaks
NetBSD's scheduler was recently changed to better distribute load of long-running processes on multiple CPUs. So far, the associated sysctl tweaks were not documented, and this was changed now, documenting the kern.sched sysctls.

For reference, here is the text that was added to the sysctl(7) manpage:

     kern.sched (dynamic)
             Influence the scheduling of LWPs, their priorisation and how they
             are distributed on and moved between CPUs.

                   Third level name              Type       Changeable
                   kern.sched.cacheht_time       integer    yes
                   kern.sched.balance_period     integer    yes
                   kern.sched.average_weight     integer    yes
                   kern.sched.min_catch          integer    yes
                   kern.sched.timesoftints       integer    yes
                   kern.sched.kpreempt_pri       integer    yes
                   kern.sched.upreempt_pri       integer    yes
                   kern.sched.maxts              integer    yes
                   kern.sched.mints              integer    yes
                   kern.sched.name               string     no
                   kern.sched.rtts               integer    no
                   kern.sched.pri_min            integer    no
                   kern.sched.pri_max            integer    no

             The variables are as follows:

             kern.sched.cacheht_time (dynamic)
                     Cache hotness time in which a LWP is kept on one particu-
                     lar CPU and not moved to another CPU. This reduces the
                     overhead of flushing and reloading caches.  Defaults to
                     3ms.  Needs to be given in ``hz'' units, see mstohz(9).

             kern.sched.balance_period (dynamic)
                     Interval at which the CPU queues are checked for re-bal-
                     ancing.  Defaults to 300ms.  Needs to be given in ``hz''
                     units, see mstohz(9).

             kern.sched.average_weight (dynamic)
                     Can be used to influence how likely LWPs are to be
                     migrated from one CPU's queue of LWPs that are ready to
                     run to a different, idle CPU.  The value gives the per-
                     centage for weighting the average count of migratable
                     threads from the past against the current number of
                     migratable threads.  A small value gives more weight to
                     the past, a larger values more weight on the current sit-
                     uation.  Defaults to 50 and must be between 0 and 100.

             kern.sched.min_catch (dynamic)
                     Minimum count of migratable (runable) threads for catch-
                     ing (stealing) from another CPU.  Defaults to 1 but can
                     be increased to decrease chance of thread migration
                     between CPUs.

             kern.sched.timesoftints (dynamic)
                     Enable tracking of CPU time for soft interrupts as part
                     of a LWP's real execution time.  Set to a non-zero value
                     to enable, and see ps(1) for printing CPU times.

             kern.sched.kpreempt_pri (dynamic)
                     Minimum priority to trigger kernel preemption.

             kern.sched.upreempt_pri (dynamic)
                     Minimum priority to trigger user preemption.

             kern.sched.maxts (dynamic)
                     Scheduler specific maximal time quantum (in millisec-
                     onds).  Must be set to a value larger than ``mints'' and
                     between 10 and ``hz'' as given by the kern.clockrate
                     sysctl.  Provided by the M2 scheduler.

             kern.sched.mints (dynamic)
                     Scheduler specific minimal time quantum (in millisec-
                     onds).  Must be set to a value smaller than ``maxts'' and
                     between 1 and ``hz'' as given by the ``kern.clockrate''
                     sysctl.  Provided by the M2 scheduler.

             kern.sched.name (dynamic)
                     Scheduler name.  Provided both by the M2 and the 4BSD
                     scheduler.

             kern.sched.rtts (dynamic)
                     Fixed scheduler specific round-robin time quantum in mil-
                     liseconds.  Provided both by the M2 and the 4BSD sched-
                     uler.

             kern.sched.pri_min (dynamic)
                     Minimal POSIX real-time priority.  See sched(3).

             kern.sched.pri_max (dynamic)
                     Maximal POSIX real-time priority.  See sched(3).


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