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.