Variations
There are many possible variations of temporal multithreading, but most can be classified into two sub-forms: coarse-grained and fine-grained.
- In coarse-grained temporal multithreading, the main processor pipeline contains only one thread at a time. The processor must effectively perform a rapid context switch before executing a different thread. This fast context switch is sometimes referred to as a thread switch. There may or may not be additional penalty cycles when switching.
- There are many possible variations of coarse-grained temporal multithreading, mainly concerning the algorithm that determines when thread switching occurs. This algorithm may be based on one or more of many different factors, including cycle counts, cache misses, and fairness.
- In fine-grained temporal multithreading, the main processor pipeline may contain multiple threads, with context switches effectively occurring between pipe stages (e.g. in the barrel processor). This form of multithreading can be more expensive than the coarse-grained forms because execution resources that span multiple pipe stages may have to deal with multiple threads. Also contributing to cost is the fact that this design cannot be optimized around the concept of a "background" thread — any of the concurrent threads implemented by the hardware might require its state to be read or written on any cycle.
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