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.