[CentOS] [OT] Memory Models and Multi/Virtual-Cores -- WAS: 4.0-> 4.1 update failing

[Bryan J. Smith <b.j.smith@ieee.org>]

From: Robert Hanson <roberth at abbacomm.net>
> 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 at ieee.org