Question Zen 6 Speculation Thread

[MS_AT]

virtual core

There are no "virtual" cores. There are no "real" cores. There are just cores and some of them happen to be able to support two hardware execution contexts (independant instruction streams, however you want to call it) at the same time. What I mean to say is, that core0 is no more or less real than core1 in SMT enabled system. Neither of them is in any way preffered.

The big deal with disabling SMT was that it generated less heat per core, helping to maintain higher clocks in thermally or power limited situations.

Any measurements to link? I just wonder how significant is the clock delta and what was the actual software in the cases you had in mind.

 

[CouncilorIrissa]

There are no "virtual" cores. There are no "real" cores. There are just cores and some of them happen to be able to support two hardware execution contexts (independant instruction streams, however you want to call it) at the same time. What I mean to say is, that core0 is no more or less real than core1 in SMT enabled system. Neither of them is in any way preffered.

I'd also add there's little gain in thread-to-thread latency between sibling threads on the same core rather than threads on different physical cores. It used to be thrice as fast on Zen 4, but that's not the case anymore.

 

[igor_kavinski]

I'd also add there's little gain in thread-to-thread latency between sibling threads on the same core rather than threads on different physical cores. It used to be thrice as fast on Zen 4, but that's not the case anymore.
View attachment 126105

Isn't that an old chart without the cross CCD latency fix?

 

[CouncilorIrissa]

Isn't that an old chart without the cross CCD latency fix?

Doesn't change anything for the sibling threads.
edit: 1.2.0.2b AGESA result:

 

[Io Magnesso]

Yup. Cache pressure is the deal killer for SMT. Need bigger, faster caches for better SMT. I think that's why IBM went with the virtual cache scheme: https://www.anandtech.com/show/16924/did-ibm-just-preview-the-future-of-caches

It may be less effective than increasing the amount of cash fundamentally...
Better scheduling may also be used