xref: /freebsd/share/man/man4/man4.i386/pae.4 (revision b5e7e9995091a8dbcf193564b2ceaf886252fc9b)
1.\"
2.\" Copyright (c) 2003 Networks Associates Technology, Inc.
3.\" All rights reserved.
4.\"
5.\" This software was developed for the FreeBSD Project by Jake Burkholder,
6.\" Safeport Network Services, and Network Associates Laboratories, the
7.\" Security Research Division of Network Associates, Inc. under
8.\" DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the DARPA
9.\" CHATS research program.
10.\"
11.\" Redistribution and use in source and binary forms, with or without
12.\" modification, are permitted provided that the following conditions
13.\" are met:
14.\" 1. Redistributions of source code must retain the above copyright
15.\"    notice, this list of conditions and the following disclaimer.
16.\" 2. Redistributions in binary form must reproduce the above copyright
17.\"    notice, this list of conditions and the following disclaimer in the
18.\"    documentation and/or other materials provided with the distribution.
19.\"
20.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
23.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
30.\" SUCH DAMAGE.
31.\"
32.\" $FreeBSD$
33.\"
34.Dd April 8, 2003
35.Dt PAE 4 i386
36.Os
37.Sh NAME
38.Nm PAE
39.Sh SYNOPSIS
40.Cd "options PAE"
41.Sh DESCRIPTION
42The
43.Dv PAE
44option provides support for the physical address extensions capability
45of the
46.Tn Intel
47.Tn Pentium Pro
48and above CPUs,
49and allows for up to 64 gigabytes of memory to be used in systems capable
50of supporting it.
51With the
52.Dv PAE
53option, memory above 4 gigabytes is simply added to the general page pool.
54The system makes no distinction between memory above or below 4 gigabytes,
55and no specific facility is provided for a process or the kernel to access
56more memory than they would otherwise be able to access, through a sliding
57window or otherwise.
58.Sh SEE ALSO
59.Xr smp 4 ,
60.Xr tuning 7 ,
61.Xr config 8 ,
62.Xr bus_dma 9
63.Sh HISTORY
64The
65.Dv PAE
66option first appeared in
67.Fx 5.1 .
68.Sh AUTHORS
69.An Jake Burkholder Aq jake@FreeBSD.org
70.Sh BUGS
71Since KLD modules are not compiled with the same options headers that
72the kernel is compiled with,
73they must not be loaded into a kernel compiled with the
74.Dv PAE
75option.
76.Pp
77Many devices or their device drivers are not capable of direct memory access
78to physical addresses above 4 gigabytes.
79In order to make use of direct memory access IO in a system with more than
804 gigabytes of memory when the
81.Dv PAE
82option is used,
83these drivers must use a facility for remapping or substituting physical
84memory which is not accessible to the device.
85One such facility is provided by the
86.Nm busdma
87interface.
88Device drivers which do not account for such devices will not work reliably
89in a system with more than 4 gigabytes of memory when the
90.Dv PAE
91option is used,
92and may cause data corruption.
93The
94.Pa PAE
95kernel configuration file includes the
96.Dv PAE
97option, and explicitly excludes all device drivers which are known to not work
98or have not been tested in a system with the
99.Dv PAE
100option and more than 4 gigabytes of memory.
101.Pp
102Many parameters which determine how memory is used in the kernel are based on
103the amount of physical memory.
104The formulas used to determine the values of these paramters for specific
105memory configurations may not take into account the fact there may be more
106than 4 gigabytes of memory, and may not scale well to these memory
107configurations.
108In particular,
109it may be necessary to increase the amount of virtual address space available
110to the kernel,
111or to reduce the amount of a specific resource that is heavily used,
112in order to avoid running out of virtual address space.
113The
114.Dv KVA_PAGES
115option may be used to increase the kernel virtual address space,
116and the
117.Va kern.maxvnodes
118.Xr sysctl 8
119may be used to decrease the number of vnodes allowed,
120an example of a resource that the kernel is likely to overallocate in
121large memory configurations.
122For optimal performance and stability it may be necessary to consult the
123.Xr tuning 7
124manual page, and make adjustments to the parameters documented there.
125