128 lines
4.6 KiB
Groff
128 lines
4.6 KiB
Groff
.\"
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.\" Copyright (c) 2003 Networks Associates Technology, Inc.
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.\" All rights reserved.
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.\"
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.\" This software was developed for the FreeBSD Project by Jake Burkholder,
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.\" Safeport Network Services, and Network Associates Laboratories, the
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.\" Security Research Division of Network Associates, Inc. under
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.\" DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the DARPA
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.\" CHATS research program.
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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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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR 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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.\" $FreeBSD$
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.\"
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.Dd April 8, 2003
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.Dt PAE 4 i386
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.Os
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.Sh NAME
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.Nm PAE
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.Nd Physical Address Extensions
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.Sh SYNOPSIS
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.Cd "options PAE"
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.Sh DESCRIPTION
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The
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.Dv PAE
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option provides support for the physical address extensions capability
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of the
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.Tn Intel
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.Tn Pentium Pro
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and above CPUs,
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and allows for up to 64 gigabytes of memory to be used in systems capable
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of supporting it.
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With the
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.Dv PAE
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option, memory above 4 gigabytes is simply added to the general page pool.
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The system makes no distinction between memory above or below 4 gigabytes,
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and no specific facility is provided for a process or the kernel to access
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more memory than they would otherwise be able to access, through a sliding
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window or otherwise.
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.Sh SEE ALSO
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.Xr smp 4 ,
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.Xr tuning 7 ,
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.Xr config 8 ,
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.Xr bus_dma 9
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.Sh HISTORY
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The
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.Dv PAE
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option first appeared in
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.Fx 4.9
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and
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.Fx 5.1 .
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.Sh AUTHORS
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.An Jake Burkholder Aq Mt jake@FreeBSD.org
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.Sh BUGS
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Since KLD modules are not compiled with the same options headers that
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the kernel is compiled with,
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they must not be loaded into a kernel compiled with the
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.Dv PAE
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option.
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.Pp
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Many devices or their device drivers are not capable of direct memory access
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to physical addresses above 4 gigabytes.
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In order to make use of direct memory access IO in a system with more than
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4 gigabytes of memory when the
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.Dv PAE
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option is used,
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these drivers must use a facility for remapping or substituting physical
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memory which is not accessible to the device.
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One such facility is provided by the
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.Nm busdma
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interface.
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Device drivers which do not account for such devices will not work reliably
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in a system with more than 4 gigabytes of memory when the
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.Dv PAE
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option is used,
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and may cause data corruption.
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The
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.Pa PAE
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kernel configuration file includes the
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.Dv PAE
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option, and explicitly excludes all device drivers which are known to not work
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or have not been tested in a system with the
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.Dv PAE
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option and more than 4 gigabytes of memory.
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.Pp
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Many parameters which determine how memory is used in the kernel are based on
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the amount of physical memory.
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The formulas used to determine the values of these parameters for specific
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memory configurations may not take into account the fact there may be more
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than 4 gigabytes of memory, and may not scale well to these memory
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configurations.
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In particular,
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it may be necessary to increase the amount of virtual address space available
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to the kernel,
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or to reduce the amount of a specific resource that is heavily used,
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in order to avoid running out of virtual address space.
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The
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.Dv KVA_PAGES
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option may be used to increase the kernel virtual address space,
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and the
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.Va kern.maxvnodes
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.Xr sysctl 8
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may be used to decrease the number of vnodes allowed,
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an example of a resource that the kernel is likely to overallocate in
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large memory configurations.
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For optimal performance and stability it may be necessary to consult the
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.Xr tuning 7
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manual page, and make adjustments to the parameters documented there.
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