/share/man/man4/man4.i386/pae.4

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 32.\" $FreeBSD$
 33.\"
 34.Dd April 8, 2003
 35.Dt PAE 4 i386
 36.Os
 37.Sh NAME
 38.Nm PAE
 39.Nd Physical Address Extensions
 40.Sh SYNOPSIS
 41.Cd "options PAE"
 42.Sh DESCRIPTION
 43The
 44.Dv PAE
 45option provides support for the physical address extensions capability
 46of the
 47.Tn Intel
 48.Tn Pentium Pro
 49and above CPUs,
 50and allows for up to 64 gigabytes of memory to be used in systems capable
 51of supporting it.
 52With the
 53.Dv PAE
 54option, memory above 4 gigabytes is simply added to the general page pool.
 55The system makes no distinction between memory above or below 4 gigabytes,
 56and no specific facility is provided for a process or the kernel to access
 57more memory than they would otherwise be able to access, through a sliding
 58window or otherwise.
 59.Sh SEE ALSO
 60.Xr smp 4 ,
 61.Xr tuning 7 ,
 62.Xr config 8 ,
 63.Xr bus_dma 9
 64.Sh HISTORY
 65The
 66.Dv PAE
 67option first appeared in
 68.Fx 4.9
 69and
 70.Fx 5.1 .
 71.Sh AUTHORS
 72.An Jake Burkholder Aq jake@FreeBSD.org
 73.Sh BUGS
 74Since KLD modules are not compiled with the same options headers that
 75the kernel is compiled with,
 76they must not be loaded into a kernel compiled with the
 77.Dv PAE
 78option.
 79.Pp
 80Many devices or their device drivers are not capable of direct memory access
 81to physical addresses above 4 gigabytes.
 82In order to make use of direct memory access IO in a system with more than
 834 gigabytes of memory when the
 84.Dv PAE
 85option is used,
 86these drivers must use a facility for remapping or substituting physical
 87memory which is not accessible to the device.
 88One such facility is provided by the
 89.Nm busdma
 90interface.
 91Device drivers which do not account for such devices will not work reliably
 92in a system with more than 4 gigabytes of memory when the
 93.Dv PAE
 94option is used,
 95and may cause data corruption.
 96The
 97.Pa PAE
 98kernel configuration file includes the
 99.Dv PAE
100option, and explicitly excludes all device drivers which are known to not work
101or have not been tested in a system with the
102.Dv PAE
103option and more than 4 gigabytes of memory.
104.Pp
105Many parameters which determine how memory is used in the kernel are based on
106the amount of physical memory.
107The formulas used to determine the values of these parameters for specific
108memory configurations may not take into account the fact there may be more
109than 4 gigabytes of memory, and may not scale well to these memory
110configurations.
111In particular,
112it may be necessary to increase the amount of virtual address space available
113to the kernel,
114or to reduce the amount of a specific resource that is heavily used,
115in order to avoid running out of virtual address space.
116The
117.Dv KVA_PAGES
118option may be used to increase the kernel virtual address space,
119and the
120.Va kern.maxvnodes
121.Xr sysctl 8
122may be used to decrease the number of vnodes allowed,
123an example of a resource that the kernel is likely to overallocate in
124large memory configurations.
125For optimal performance and stability it may be necessary to consult the
126.Xr tuning 7
127manual page, and make adjustments to the parameters documented there.