A disadvantage in running Linux, on a multi-core CPU that’s threaded.

One of the facts about modern computing is, that the hardware could include a multi-core CPU, with a number of virtual cores different from the number of full cores. Such CPUs were once called “Hyper-Threaded”, but are now only called “Threaded”.

If the CPU has 8 virtual cores, but is threaded as only 4 full cores, then there will only be a speed advantage, when running 4 processes. But because processes are sometimes multi-threaded, each of those 4 processes could consist of 2 fully-busy threads, and benefit from a further doubling of speed because each full core has 2 virtual cores.

It’s really a feature of Windows to exploit this fully, while Linux tends to ignore this. When Linux runs on such a CPU, it only ‘sees’ the maximum number of virtual cores, as the logical number of cores that the hardware has, without taking into account that they could be pairing in some way, to result in a lower number of full cores.

And to a certain extent, the Linux kernel is justified in doing so because unlike how it is with Windows, it’s actually just as cheap for a Linux computer to run a high number of separate processes, as it is to run processes with the same number of threads. Two threads share a code segment as well as a data segment (heap), but have two separate stack segments as well as different register-values. This makes them ‘enlightened processes’. Well they only really run faster under Windows (or maybe under OS/X).

Under Linux it’s fully feasible just to create many processes instead, so the bulk of the programming work does not make use as much of multi-threading. Of course Even under Linux, code is sometimes written to be multi-threaded, for reasons I won’t go into here.

But then under Linux, there was also never effort put into the kernel recognizing two of its logical cores, as belonging to the same full core.

(Updated 2/19/2019, 17h30 … )

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