128 lines
5.0 KiB
Groff
128 lines
5.0 KiB
Groff
.\" $NetBSD: scheduler.9,v 1.3 2003/04/16 13:35:33 wiz Exp $
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.\"
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.\" Copyright (c) 2002 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Gregory McGarry.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd September 21, 2002
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.Dt SCHEDULER 9
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.Os
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.Sh NAME
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.Nm scheduler
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.Nd process scheduling subsystem
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.Sh SYNOPSIS
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.In sys/sched.h
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.Sh DESCRIPTION
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.Bf -symbolic
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This page documents the default
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.Dq time-sharing
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scheduler in
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.Nx .
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It is likely that in the future a new scheduler subsystem will be
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developed which will allow new scheduling algorithms to be
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implemented.
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.Ef
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.Pp
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.Nx Ns 's
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scheduler algorithm is based on
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.Dq multilevel feedback queues .
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All processes that are runnable are assigned a scheduling priority
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that determines in which run queue they are placed.
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The currently running process does not exist on a run queue.
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In selecting a new process to run, the system scans the run queues
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from highest to lowest priority and chooses the first process on the
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first non-empty queue.
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If multiple processes reside on a queue, the system runs them round
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robin in the order found on the queue.
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If a process uses up the time slice allocated to it, it is placed at
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the end of the queue from which it came, and the process at the front of
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the queue is selected to run.
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.Pp
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The system adjusts the scheduling priority of a process dynamically to
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reflect resource requirements (being blocked) and the CPU utilization
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consumed by the process.
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Processes are moved between run queues based on changes in their
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scheduling priority (hence the word feedback in the name).
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.Pp
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The process CPU utilization is incremented in
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.Fn hardclock
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each time that the system clock ticks and the process is found to be
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executing.
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CPU utilization is adjusted once per second in
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.Fn schedcpu
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via a digital decay filter.
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Each time a process accumulates four ticks in its CPU utilization,
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.Fn schedclock
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invokes
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.Fn resetpriority
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to recalculate the process's scheduling priority.
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.Pp
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When a process other than the currently running process attains a
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higher scheduling priority, the system notices this in
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.Fn resetpriority
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and schedules a context switch to the higher-priority process to occur
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on return to user mode.
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.Pp
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A blocked process exists on the sleep queue and is not affected by the
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scheduling behaviour discussed above.
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However, each second
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.Fn schedcpu
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updates the time each process has been on the sleep queue.
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The system recomputes a process's CPU utilization estimate in
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.Fn updatepri
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when it is awakened and has been sleeping for longer than 1 second.
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When the process is placed on the run queue by
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.Fn setrunnable ,
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.Fn resetpriority
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is invoked to recalculate the process's scheduling priority.
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.Pp
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The system tailors this short-term scheduling algorithm to favor
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interactive jobs by raising the scheduling priority of processes that
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are blocked waiting for I/O for one or more seconds, and by lowering the
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priority of processes that accumulate significant amounts of CPU time.
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.Sh CODE REFERENCES
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This section describes places within the
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.Nx
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source tree where actual code implementing the scheduler can be found.
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All pathnames are relative to
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.Pa /usr/src .
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.Pp
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The scheduler subsystem is implemented within the file
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.Pa sys/kern/kern_synch.c .
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.Sh SEE ALSO
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.Xr SCHED_LOCK 9 ,
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.Xr hardclock 9 ,
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.Xr mi_switch 9 ,
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.Xr setrunqueue 9 ,
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.Xr userret 9
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