Simultaneous multithreading

Simultaneous multithreading (SMT) is a technique for improving the overall efficiency of superscalar CPUs with hardware multithreading. SMT permits multiple independent threads of execution to better utilize the resources provided by modern processor architectures.

Details

The name multithreading is obfuscating, because not only can multiple threads be executed simultaneously on one CPU core, but also multiple tasks (with different Page tables, different Task state segments, different Protection rings, different IO permissions, ...). Although running on the same core, they are completely separated from each other. Multithreading is similar in concept to preemptive multitasking but is implemented at the thread level of execution in modern superscalar processors.

Simultaneous multithreading (SMT) is one of the two main implementations of multithreading, the other form being temporal multithreading. In temporal multithreading, only one thread of instructions can execute in any given pipeline stage at a time. In simultaneous multithreading, instructions from more than one thread can be executed in any given pipeline stage at a time. This is done without great changes to the basic processor architecture: the main additions needed are the ability to fetch instructions from multiple threads in a cycle, and a larger register file to hold data from multiple threads. The number of concurrent threads can be decided by the chip designers. Two concurrent threads per CPU core are common, but some processors support eight concurrent threads per core.