
Marvell Details ThunderX3 CPUs - StillBored
https://www.anandtech.com/print/15995/hot-chips-2020-marvell-details-thunderx3
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sfshaw
SMT4 is a bit of a surprise here. Obviously POWER uses it well but this feels
like new territory for ARM designs.

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rwmj
SMT in general is the surprise. Is there any other ARM core out there with
multiple threads? Of course once you are able to do multithreading then adding
more contexts (4 instead of the 2 common on x86) is "just" a matter of finding
the right trade-off between resources and memory latency.

Edit: Apparently the Cortex A65AE ("AE" = automotive enhanced) was released
last year and was the first ARM core with multithreading.

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person_of_color
Huh? Don't most ARM SoCs have multiple cores, where threads can be scheduled -
effectively simultaneous multithreading?

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notacoward
A "thread" in SMT is not the kind of OS-level or user-level thread most people
are used to. Think of N threads as N register sets[1] that are swapped in and
out _on the same core_ , mostly to hide memory/cache latency. There's still
only one set of functional units, and only one thread can be active on the
core at a time - unlike separate cores which can all be active simultaneously.
"Time-shared multithreading" or "multiplexed cores" might be more accurate,
but SMT has been the established term at least since Tera.

[1] It's actually more complicated than that, with register aliasing etc, but
it's a decent conceptual model.

