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Comment by tgma | Hacker News Reader

The way they refer to cores in their system is confusing and non-standard. The author talks about a 5900X as a 24 core machine and discusses as if there are 24 cores, 12 of which are piggybacking on the other 12. In reality, there are 24 hyperthreads that are pretty much pairwise symmetric that execute on top of 12 cores with two sets of instruction pipeline sharing same underlying functional units.

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Years ago, when trying to explain hyper threading to my brother, who doesn't have any specialized technical knowledge, he came up with the analogy that it's like 2-ply toilet paper. You don't quite have 24 distinct things, but you have 12 that are roughly twice as useful as the individual ones, although you can't really separate them and expect them to work right.

Nah, it's easier than that. Putting two chefs in the same kitchen doesn't let you cook twice the amount of food in the same amount of time, because sometimes the two chefs need to use the same resource at the same time - e.g. sink, counter space, oven. But, the additional chef does improve the utilization of the kitchen equipment, leaving fewer things unused.

Maybe simplify more to make the concept of shared resource explicit.

2 chefs with one stove. As long as they're doing other things than frying it's ok and speeding things up but once they both need the stove you're down to 1 working and 1 waiting.

That’s perfect!

Especially when it come to those advertisements “6 large rolls == 18 normal rolls”.

Sure it might be thicker but nobody wipes their butt with 1/3 a square…

> he came up with the analogy that it's like 2-ply toilet paper.

as in youd only use it to wipe excrement from around your sphincter

Thanks for the feedback. I think you're right, so I changed a bunch of references and updated the description of the processor to 12 core / 24 thread. In some cases, I still think "cores" is the right terminology though, since my OS (confusingly) reports utilization as-if I had 24 cores.

Eh, what’s a thread really? It’s a term for us humans.

The difference between two threads and one core or two cores with shared resources?

Nothing is really all that neat and clean.

It more of a 2 level NUMA type architecture with 2 sets of 6 SMP sets of 2.

The scheduler may look at it that way (depending), but to the end user? Or even to most of the system? Nah.

There are observable differences. For example, under HT, TLB flush or context switch will likely be observable by a neighboring thread whereas for in a full dedicated core, you won't observe such things.

Will be interesting when (if?) Intel ships software defined cores which are the logical inverse of hyper threading.

Instead of having a big core with two instruction pipelines sharing big ALUs etc, they have two (or more) cores that combine resources and become one core.

Almost the same, yet quite different.

https://patents.google.com/patent/EP4579444A1/en

There was the dreaded AMD FX chip which was advertised as 8 core, but shared functional units. Got sued, etc.

That patent seems to be describing a dumb way to implement pipelining / speculative execution. Am I missing something?

Anyway, by my reading, it’s also similar to the Itanic, er, Itanium, where the “cores” that got combined were pipeline stages.

I did not read the patent (do not read patents as a matter of policy.) Was simply responding to the second paragraph that kind of reminded me of FX Bulldozer chips.

If both SMT cores are being asked to do the same workload they will likely contend for the same resource and execution units internally so the boost from SMT will be less. If they have different workloads the boost will be more. Now throw in P and E cores on newer CPU's, turbo and non-turbo, everything gets very complicated. I did see a study that adding SMT got a much better performance per watt boost than adding turbo which was interesting/useful.

Worth noting that the major clouds will sell this as 24 "vcpus".

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