PAE —
Physical
Address Extensions
options PAE
The
PAE option provides support for the
physical address extensions capability of the Intel 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
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.
smp(4),
tuning(7),
config(8),
bus_dma(9)
The
PAE option first appeared in
FreeBSD 4.9 and
FreeBSD 5.1.
Jake Burkholder
<
[email protected]>
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
PAE option.
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
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
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
PAE option is
used, and may cause data corruption. The
PAE
kernel configuration file includes the
PAE
option, and explicitly excludes all device drivers which are known to not work
or have not been tested in a system with the
PAE option and more than 4 gigabytes of
memory.
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
KVA_PAGES
option may be used to increase the kernel virtual address space, and the
kern.maxvnodes
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
tuning(7) manual page, and make adjustments to
the parameters documented there.