/share/man/man4/man4.i386/pae.4
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- .\" Copyright (c) 2003 Networks Associates Technology, Inc.
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- .\" This software was developed for the FreeBSD Project by Jake Burkholder,
- .\" Safeport Network Services, and Network Associates Laboratories, the
- .\" Security Research Division of Network Associates, Inc. under
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- .Dd April 8, 2003
- .Dt PAE 4 i386
- .Os
- .Sh NAME
- .Nm PAE
- .Nd Physical Address Extensions
- .Sh SYNOPSIS
- .Cd "options PAE"
- .Sh DESCRIPTION
- The
- .Dv PAE
- option provides support for the physical address extensions capability
- of the
- .Tn Intel
- .Tn Pentium Pro
- and above CPUs,
- and allows for up to 64 gigabytes of memory to be used in systems capable
- of supporting it.
- With the
- .Dv PAE
- option, memory above 4 gigabytes is simply added to the general page pool.
- The system makes no distinction between memory above or below 4 gigabytes,
- and no specific facility is provided for a process or the kernel to access
- more memory than they would otherwise be able to access, through a sliding
- window or otherwise.
- .Sh SEE ALSO
- .Xr smp 4 ,
- .Xr tuning 7 ,
- .Xr config 8 ,
- .Xr bus_dma 9
- .Sh HISTORY
- The
- .Dv PAE
- option first appeared in
- .Fx 4.9
- and
- .Fx 5.1 .
- .Sh AUTHORS
- .An Jake Burkholder Aq jake@FreeBSD.org
- .Sh BUGS
- Since KLD modules are not compiled with the same options headers that
- the kernel is compiled with,
- they must not be loaded into a kernel compiled with the
- .Dv PAE
- option.
- .Pp
- Many devices or their device drivers are not capable of direct memory access
- to physical addresses above 4 gigabytes.
- In order to make use of direct memory access IO in a system with more than
- 4 gigabytes of memory when the
- .Dv PAE
- option is used,
- these drivers must use a facility for remapping or substituting physical
- memory which is not accessible to the device.
- One such facility is provided by the
- .Nm busdma
- interface.
- Device drivers which do not account for such devices will not work reliably
- in a system with more than 4 gigabytes of memory when the
- .Dv PAE
- option is used,
- and may cause data corruption.
- The
- .Pa PAE
- kernel configuration file includes the
- .Dv PAE
- option, and explicitly excludes all device drivers which are known to not work
- or have not been tested in a system with the
- .Dv PAE
- option and more than 4 gigabytes of memory.
- .Pp
- Many parameters which determine how memory is used in the kernel are based on
- the amount of physical memory.
- The formulas used to determine the values of these parameters for specific
- memory configurations may not take into account the fact there may be more
- than 4 gigabytes of memory, and may not scale well to these memory
- configurations.
- In particular,
- it may be necessary to increase the amount of virtual address space available
- to the kernel,
- or to reduce the amount of a specific resource that is heavily used,
- in order to avoid running out of virtual address space.
- The
- .Dv KVA_PAGES
- option may be used to increase the kernel virtual address space,
- and the
- .Va kern.maxvnodes
- .Xr sysctl 8
- may be used to decrease the number of vnodes allowed,
- an example of a resource that the kernel is likely to overallocate in
- large memory configurations.
- For optimal performance and stability it may be necessary to consult the
- .Xr tuning 7
- manual page, and make adjustments to the parameters documented there.