PAE
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 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 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.
FreeBSD 5.2.1 April 8, 2003 FreeBSD 5.2.1 [ Back ] |
