Simultaneous multithreading (SMT)
SMT fetch unit can take advantage of the interthread competition for instruction bandwidth in two ways:
- First, it can partition fetch bandwidth among the threads and fetch from several threads each cycle. Goal: increasing the probability of fetching only non speculative instructions.
- Second, the fetch unit can be selective about which threads it fetches.
The main drawback to simultaneous multithreading may be that it complicates the instruction issue stage, which always is central to the multiple threads.
A functional partitioning as demanded for processors of the 109-transistor era is therefore not easily reached.
No simultaneous multithreaded processors exist to date. Only simulations.
General opinion: SMT will be in next generation microprocessors.
Announcement (Oct. 1999): Compaq Alpha 21464 (EV8) will be four-threaded SMT