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+.\"     $NetBSD: sched_4bsd.9,v 1.9 2019/04/09 13:53:52 sevan Exp $
+.\"
+.\" Copyright (c) 2002 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" This code is derived from software contributed to The NetBSD Foundation
+.\" by Gregory McGarry and Daniel Sieger.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.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