Perhaps you’re thinking about SMT (e.g. Intel Hyper Threading) when you say “proper multithreading”?
I’m not sure it’s valid to say that only SMT is “proper multithreading”, especially since multithreading as a concept predates it by quite a way.
SMT has a quite a few performance issues since resources such as the L1, L2, and branch predictor are shared between the threads, which can lead to contention that hurts the performance of all the SMT threads sharing a physical core.
SMP is no less “proper”, and as core counts have increased significantly on commodity CPUs, the use of spinning threads bound to a single core each has become a common paradigm.
Oversubscription without SMT (i.e. many threads per core) is possible, but unless you have a workload where each thread is I/O bound with a substantial amount of time spent blocking, the overhead of scheduling and context switching means throughput will likely decrease.
All SMT does is allow multiple instruction counters on the same superscalar core. It increases utilization of all compute units and therefore increases throughput.
Of course it increases latency, since those resources are not fully exclusive to a particular thread anymore.
Whether or not it's a good thing depends on what you care about. You could also argue that a good program would be able to saturate a single superscalar core with a single thread and thus wouldn't benefit from SMT at all, but I think that would be hard to guarantee in practice.
I’m not sure it’s valid to say that only SMT is “proper multithreading”, especially since multithreading as a concept predates it by quite a way.
SMT has a quite a few performance issues since resources such as the L1, L2, and branch predictor are shared between the threads, which can lead to contention that hurts the performance of all the SMT threads sharing a physical core.
SMP is no less “proper”, and as core counts have increased significantly on commodity CPUs, the use of spinning threads bound to a single core each has become a common paradigm.
Oversubscription without SMT (i.e. many threads per core) is possible, but unless you have a workload where each thread is I/O bound with a substantial amount of time spent blocking, the overhead of scheduling and context switching means throughput will likely decrease.