If you're got a workload that isn't characterized by missing cache and waiting around for main memory fairly often then usually SMT is going to be pretty ineffi

If you're got a workload that isn't characterized by missing cache and waiting around for main memory fairly often then usually SMT is going to be pretty inefficient. As a crude rule of thumb, the single thread performance you get is going to scale as something like the square root of the number of transistors you throw at the problem. So if you want two threads of a certain performance you can either use one bit core with SMT-2 or two cores each a third or so the size. And the two cores will tend to be more power efficient two with less data moving long distances through the caches. Now, if you are hitting main memory often it makes sense to go wild with thread and use SMT-8 like IBM's POWER cores or Sun's SPARC cores did. And if you mostly just care about maxing out single threaded performance for user responsiveness then you do indeed have lots of unused resources most of the time and you might as well add SMT for when you're more throughput bound. But while the design and transistor costs of adding SMT to a design are very modest, everything I've heard about the test and verification of it seems pretty hairy.