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Re: SMP does not work as expected - was: Re: [Xen-users] Checking for SMP usage in xen stable

On Mon, 7 Nov 2005, Dirk H. Schulz wrote:

> Tom Brown schrieb:
> >this is just the normal P4/xeon hyperthreading isn't it? I don't know what
> >an IBM x335 is... so I'm just guessing that hyperthreading is available.
> >
> >
> Yes, this is a Xeon that can do hyperthreading - but does SMP in Xen
> also work on hyperthreaded single cpus? I thought that hyperthreading is
> different from SMP.

it is different ... hyperthreading only "looks" like SMP. I am no expert
on it, but ran across something recently that indicated it was something
Intel introduced to reduce the effect of the deep pipeline in their
processors.  The idea is that the processor can be working on two
"programs" at the same time, and if the data or instructions for one of
the programs is not available, the other one might be ready and able to
make productive use of the CPU core.

> If that works I can distribute CPU power in finer chunks on a dual
> processor hyperthreading machine.

My understanding is that in the context of "task switching", it _is_
usefull, I would assume it is like having an extra set of registers. I
believe the Xen docs suggest that you dedicate one thread of one CPU to


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