Number of processor cores confusion

[Liam Proven]

On Mon, 9 Nov 2020 at 19:06, Robert Heller <heller at deepsoft.com> wrote:
>
> Intel developed a "hack" where one "core" had some parts of the core elements
> duplicated, allowing a level of multiprocessing.

That's a bit harsh.

• It's not just Intel -- quite a few big RISC chips do this too, for example
• It's called simultaneous multithreading:
https://en.wikipedia.org/wiki/Simultaneous_multithreading
• "Hyper-threading" is just Intel's brand name
• It's a consequence of superscalar CPU design; parts of the CPU are
replicated multiple times and in some situations it can literally do
more than one thing at a time. The chip exposes this to software by
offering more threads of execution than there are actual cores.
• Nothing says that there have to be 2 threads per core. Sun's
UltraSPARC T-series offer up to 16 threads per core

>  Linux shows these cores as two cores (each).

The chip tells whatever OS it's running that it has _n_ cores, where
_n_ is the number of threads it can support. Linux has no say in the
matter. Usually you can turn SMT on or off in the firmware, though.

> I did not know that AMD
> also implemented this hack.

I think it's fairly new but not brand new.

> It is a bit of a hack, since the multi-threading
> does not allow for full SMP.

Depends on the task. For some things it can be a bit faster, for some
a bit slower. It tends to slightly improve system responsiveness but
that's very hard to benchmark.

And most benchmarking experts lost their jobs when all the magazines
went under, replaced by clueless kids behind websites.

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