.\"     $NetBSD: sched_4bsd.9,v 1.9 2019/04/09 13:53:52 sevan Exp $
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.\" Copyright (c) 2002 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Gregory McGarry and Daniel Sieger.
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.Dd April 9, 2019
.Dt SCHED_4BSD 9
.Os
.Sh NAME
.Nm sched_4bsd
.Nd The 4.4BSD thread scheduler
.Sh SYNOPSIS
.In sys/sched.h
.Ft void
.Fn resetpriority "lwp_t *l"
.Ft void
.Fn sched_tick "struct cpu_info *ci"
.Ft void
.Fn sched_schedclock "lwp_t *l"
.Ft void
.Fn sched_pstats_hook "struct proc *p" "int minslp"
.Ft void
.Fn sched_setrunnable "lwp_t *l"
.Ft void
.Fn updatepri "lwp_t *l"
.Sh DESCRIPTION
The traditional
.Bx 4.4
scheduler employs a
.Dq multilevel feedback queues
algorithm, favouring interactive, short-running threads to
CPU-bound ones.
.Pp
.Fn 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.
.Pp
.Fn sched_tick
gets called from
.Xr hardclock 9
every 100ms to force a switch between equal priority threads.
.Pp
The priority of the current thread is adjusted through
.Fn sched_schedclock .
The priority of a thread gets worse as it accumulates CPU time.
.Pp
.Fn sched_pstats_hook
gets called from
.Fn sched_pstats
every Hz ticks in order to recompute the priorities of all threads.
.Pp
.Fn sched_setrunnable
checks if an LWP has slept for more than one second.
If so, its priority is updated by
.Fn updatepri .
.Sh EXAMPLES
To determine the scheduler currently in use
.Bd -literal -offset indent
$ sysctl kern.sched.name
kern.sched.name = 4.4BSD
.Ed
.Sh CODE REFERENCES
The
.Bx 4.4
scheduler subsystem is implemented within the file
.Pa sys/kern/sched_4bsd.c .
.Sh SEE ALSO
.Xr csf 9 ,
.Xr hardclock 9 ,
.Xr mi_switch 9 ,
.Xr sched_m2 9 ,
.Xr userret 9
.Rs
.%A Marshall Kirk McKusick
.%A Keith Bostic
.%A Michael J. Karels
.%A John S. Quarterman
.%B "The Design and Implementation of the 4.4BSD Operating System"
.%I "Addison Wesley"
.%D 1996
.Re