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

[B Vance]

On Wed, 2006-01-25 at 12:24 -0700, Duncan wrote:
> B Vance posted <1138205784.11529.1.camel@×××××××××××.ShadowRealm>,
> excerpted below,  on Wed, 25 Jan 2006 11:16:24 -0500:
>
> >> virtual cpus?
> >>
> >> intel has hyperthreading, if you mean that, not amd.
> >>
> >> the rule of thumb is cores+1
> >>
> >> but it is totally personal taste, what you set for jX.
> >> 1,2,3,4,5 just try it and choose the number where the result is most to your
> >> liking.
> >
> > Cool, thanks.  Must of been thinking of something else that AMD had
> > licenced from Intel for core CPU use.  Could have sworn that AMD had a
> > version of Hyperthreading.
>
> Maybe you are thinking about (the unrelated) HyperTransport?  That's an
> all-AMD thing, the reason they seem to have such good performance compared
> to Intel, which is stuck with legacy front-side-bus architecture for the
> moment, with it throwing a /serious/ wrench into their upgrade plans ATM.
>
> As for Hyperthreading, AMD hasn't used it and really doesn't need or want
> to.  Hyperthreading works its magic by working around the extremely deep
> pipeline that Intel had used, in ordered to get their clock rates so high.
> Such a deep pipeline forces a *HUGE* penalty when there's a branch
> miss-prediction and the CPU has to throw away several cycles worth of work
> and let the bottom end of the pipeline idle while it loads the correct
> instructions from the top of the pipeline again.  Hyperthreading is a
> rather neat solution that avoids the severe penalty by having a second
> task ready and waiting to execute while the first one gets loaded into the
> pipeline once again.
>
> AMD has a somewhat shorter pipeline, and uses some other technologies to
> improve the branch prediction and lessen the severity of mis-prediction
> hits, both.  It isn't /that/ much shorter a pipeline, the branch
> prediction isn't /that/ much better, and the miss-prediction penalty not
> /that/ much less, but put all three together, and add in the effect of the
> front-side-bus vs hypertransport on memory latency, and the individual
> single-digit improvements begin to add-up to something rather bigger.
> Hyperthreading only yields an approximate 30% increase in performance,
> best-case, anyway, compared to a theoretical 100% increase if it were a
> real CPU.  Thus, AMD doesn't have to cover as much ground with their
> individual "minor" improvements as one might think, and because
> hyperthreading has certain downsides (effectively halving the available
> cache per thread, and certain thread-cache-access security issues where
> the hyperthread has access to the memory of the other thread), they
> believe they do better without hyperthreading.
>
> I believe AMD does better without hyperthreading as well, from a technical
> viewpoint, altho Intel has the PR game won with hyperthreading, as it's
> simply not possible to explain in two sentences why AMD doesn't need
> hyperthreading, as opposed to the relatively simple concept Intel has to
> get across, of pretending there are two CPUs so the second task can get
> some work done when the first task is waiting for something.
>
> --
> 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
>
>
Thank you for explaining that Duncan.  Guess it's time for me to start
reading a bit more then I have lately.

B. Vance

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