Marvell Details ThunderX3 CPUs | Hacker News Reader

Marvell Details ThunderX3 CPUs

anandtech.com

39 pointsStillBored9 comments

SMT4 is a bit of a surprise here. Obviously POWER uses it well but this feels like new territory for ARM designs.

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.

The ThunderX2 also has SMT4, and has been available for several years.

Indeed so it does. Even used one without spotting that.

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

SMT means multiple threads executing concurrently on a single core. Multiple cores each executing a single thread is not the same thing.

SMT is a well established teem in computer architecture and means running more than one thread per core.

SMT is multithreading within a single core. Timesharing a core at the hardware level.

Having multiple threads by having multiple cores is a different thing.

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.

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