87 lines
4.3 KiB
Perl
87 lines
4.3 KiB
Perl
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.\" Copyright (c) 1986 The Regents of the University of California.
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.\" All rights reserved.
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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 University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)0.t 5.1 (Berkeley) 4/16/91
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.\"
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.rm CM
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.TL
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A New Virtual Memory Implementation for Berkeley
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.UX
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.AU
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Marshall Kirk McKusick
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Michael J. Karels
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.AI
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Computer Systems Research Group
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Computer Science Division
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Department of Electrical Engineering and Computer Science
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University of California, Berkeley
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Berkeley, California 94720
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.AB
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With the cost per byte of memory approaching that of the cost per byte
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for disks, and with file systems increasingly distant from the host
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machines, a new approach to the implementation of virtual memory is
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necessary. Rather than preallocating swap space which limits the
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maximum virtual memory that can be supported to the size of the swap
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area, the system should support virtual memory up to the sum of the
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sizes of physical memory plus swap space. For systems with a local swap
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disk, but remote file systems, it may be useful to use some of the memory
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to keep track of the contents of the swap space to avoid multiple fetches
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of the same data from the file system.
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.PP
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The new implementation should also add new functionality. Processes
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should be allowed to have large sparse address spaces, to map files
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into their address spaces, to map device memory into their address
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spaces, and to share memory with other processes. The shared address
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space may either be obtained by mapping a file into (possibly
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different) parts of their address space, or by arranging to share
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``anonymous memory'' (that is, memory that is zero fill on demand, and
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whose contents are lost when the last process unmaps the memory) with
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another process as is done in System V.
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.PP
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One use of shared memory is to provide a high-speed
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Inter-Process Communication (IPC) mechanism between two or more
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cooperating processes. To insure the integrity of data structures
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in a shared region, processes must be able to use semaphores to
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coordinate their access to these shared structures. In System V,
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these semaphores are provided as a set of system calls. Unfortunately,
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the use of system calls reduces the throughput of the shared memory
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IPC to that of existing IPC mechanisms. We are proposing a scheme
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that places the semaphores in the shared memory segment, so that
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machines that have a test-and-set instruction can handle the usual
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uncontested lock and unlock without doing a system call. Only in
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the unusual case of trying to lock an already-locked lock or in
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releasing a wanted lock will a system call be required. The
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interface will allow a user-level implementation of the System V
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semaphore interface on most machines with a much lower runtime cost.
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.AE
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.LP
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.bp
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