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     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 ⟨


     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.