SCHED_4BSD(9)						   BSD Kernel Developer's Manual					     SCHED_4BSD(9)

NAME

     sched_4bsd -- The 4.4BSD thread scheduler

SYNOPSIS

     #include <sys/sched.h>

     void
     resetpriority(lwp_t *l);

     void
     sched_tick(struct cpu_info *ci);

     void
     sched_schedclock(lwp_t *l);

     void
     sched_pstats_hook(struct proc *p, int minslp);

     void
     sched_setrunnable(lwp_t *l);

     void
     updatepri(lwp_t *l);

DESCRIPTION

     The traditional 4.4BSD scheduler employs a ``multilevel feedback queues'' algorithm, favouring interactive, short-running threads to CPU-
     bound ones.

     resetpriority() recomputes the priority of a thread running in user mode.	If the resulting priority is higher than that of the current
     thread, a reschedule is arranged.

     sched_tick() gets called from hardclock(9) every 100ms to force a switch between equal priority threads.

     The priority of the current thread is adjusted through sched_schedclock().  The priority of a thread gets worse as it accumulates CPU time.

     sched_pstats_hook() gets called from sched_pstats() every Hz ticks in order to recompute the priorities of all threads.

     sched_setrunnable() checks if an LWP has slept for more than one second.  If so, its priority is updated by updatepri().

CODE REFERENCES

     The 4.4BSD scheduler subsystem is implemented within the file sys/kern/sched_4bsd.c.

SEE ALSO

     csf(9), hardclock(9), mi_switch(9), sched_m2(9), userret(9)

     Marshall Kirk McKusick, Keith Bostic, Michael J. Karels, and John S. Quarterman, The Design and Implementation of the 4.4BSD Operating
     System, Addison Wesley, 1996.

BSD
								   July 14, 2007							       BSD