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by thrownaway2424 4698 days ago
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.
1 comments
corresation 4698 days ago
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.

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thrownaway2424 4698 days ago
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.
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corresation 4697 days ago
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.

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