From: Robert Hanson <roberth@xxxxxxxxxxxx>
> or should i be more specific with the question(s)?
> the reason i ask is that i just dumped 2 gig dram in a basic
> P4 Intel 3.0GHz box to play with.
> regards and TIA,
At more than 1GiB on Linux/x86, you must use a 4G+4G kernel
(this is the default) to see more than 960MiB. This causes a
signficant (10%+) performance hit. On more than 4GiB, it is
worsened as more extensive paging is used.
If you have 1GiB or less, you should rebuild with_out_ "HIGHMEM"
support which is a 1G+3G kernel, and you'll see better performance
(and memory will be limited to 960MiB).
In a nutshell, you should be running Linux/x86-64 on systems with
more than 1GiB for optimal performance. If you have more than
4GiB of combined system and memory mapped I/O, you should be
running Opterons with I/O MMUs. Intel EM64T systems will have
protections in place for both earlier generation GTL+ limitations,
as well as lack of an I/O MMU.
Much of the additional "tangent" surrounded the fact that there are
a few so-called "32-bit" Athlons that actually have a BIOS hack and
Linux kernel support so it doesn't take a performance hit. Long
story short, it has to do with the fact that even so-called "32-bit"
Athlons have a core and underlying interconnect platform that
supports 40-bit _linear_ addressing _natively_.
--
Bryan J. Smith mailto:b.j.smith@xxxxxxxx
