[corresation]

I'd be humored to hear your idea of what the difference is, given the misplaced use of scare-quotes.

A hyperthread core is a virtual core -- it is not actually a core at all but is a re-purposed, possibly stalled physical core. While it can improve some scenarios, in some cases (particularly core-saturating benchmarks) it can actually hurt performance.

This is hardly an out there or controversial statement. Further I didn't say to disable hyperthreading, I said to try setting parallelism to the physical cores. Again, nothing, whatsoever, controversial about that.

[thrownaway2424]

It is an "out there" statement because it's entirely, radically incorrect. A processor thread represents a full-blown decode and issue pipeline. A core represents a set of execution resources. Each pipeline can dispatch to any execution unit equally. In case of contention for the same execution unit, one thread issues immediately and the other thread issues next.

If you don't disable hyperthreading, but instead run four threads on an 8-thread CPU, it is extremely likely that the threads will be scheduled on the first two cores/four threads and the other two cores will be shut down, especially on the newer intel CPUs with "turbo" features where this strategy can have large benefits.

[corresation]

The operating system schedules threads across cores, and the processor has zero say in the matter (further, the execution units are primarily to facilitate branch to essentially execute future scenarios). Both Linux and Windows are hyperthread aware, and will schedule threads to physical processors first, then to hyperthread virtual processors (given that it shares resources with the physical core and can sabotage performance).

This is common knowledge, and your laughable obnoxiousness, which anyone who has ever worked with multithreaded code on a HT processor knows is farce, rings pretty ridiculous.

[thrownaway2424]

No, the power-aware scheduler in Linux does not work as you describe. On a turbo-capable Intel CPU, if there are N program threads that will fit on M cores where M is less than the total cores on a socket, and the CPU will enter P0 state, then the threads will run on as few cores as possible and the remaining cores will be shut down.

[corresation]

Two threads, each running at 100%, will be assigned to two physical cores. This is reality, and is obvious given that assigning it to a physical core and a hyperthread core at most will give you about 130% instead of the 200% two physical cores will provide.

Unless, of course, you've set a power profile to prioritize power efficiency, but that would be an absolutely ridiculous assumption given that we're talking about benchmarks.