[gentoo-amd64] Re: Upgrade from single to dual core CPU

[Duncan]

Sebastian Redl posted <43D7D08D.5080902@×××××××××××.at>, excerpted below, 
on Wed, 25 Jan 2006 20:25:01 +0100:

> Matthias Wolle wrote:
>
>>You are right! AMD licenced HT-Technology from Intel.
>>AMD Dual Core CPU's uses this to fake a HT-CPU.
>>
>>
> "Fake"? Hyperthreading is just a fancy word for what AMD's "Dual Core"
> describes much better: these CPUs have multiple cores, i.e. instruction
> handlers, that share the integer, floating point etc. units. The idea is
> that, since in a conventional CPU all but one of these units are idle at
> all times, multiple cores could make better use of the CPU.

No, "dual-core" is just that, two CPUs on the same chip.  In AMD's
implementation, since AMD has the memory management unit (MMU) on-chip as
well, the two cores do share the same MMU.  Intel uses legacy
front-side-bus, with a separate MMU, and doesn't have the core-to-core
hypertransport (that's an AMD technology) either, so their implementation
is almost exactly like two CPUs that happen to be on the same chip.

Hyperthreading is just as you described -- a way for two threads to share
the same physical single-set of instruction handlers.  As I stated in an
earlier post that probably crossed in the mail with yours, Intel benefits
from hyperthreading due to their deeper pipelining, left over from when
they were still working to increase clock cycles at all costs.  AMD
apparently saw the end of the clock cycle gravy train sooner than Intel
did, and put their engineering resources into other alternatives such as
true dual core and on-chip MMUs.  As a result, while they typically run at
a lower GHz, they have a somewhat shallower instruction pipeline and other
otherwise somewhat more efficient with their cycles, so manage quite well
in actual performance even at a lower cycle count.  As well, they don't
benefit nearly as much from hyperthreading, while suffering the same
negatives, so they don't use it, choosing instead to go with a more
efficient true dual core design.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman in
http://www.linuxdevcenter.com/pub/a/linux/2004/12/22/rms_interview.html


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