freebsd-dev/share/man/man4/man4.i386/pae.4

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.Dd April 8, 2003
.Dt PAE 4 i386
.Os
.Sh NAME
.Nm PAE
.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 paramters 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.