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().

EXAMPLES
To determine the scheduler currently in use

    $ sysctl kern.sched.name
    kern.sched.name = 4.4BSD

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, John S. Quarterman, The Design and Implementation of the 4.4BSD Operating System, Addison Wesley, 1996.