Simultaneous Multithreading - Taxonomy

Taxonomy

In processor design, there are two ways to increase on-chip parallelism with less resource requirements: one is superscalar technique which tries to increase instruction level parallelism (ILP); the other is multithreading approach exploiting thread level parallelism (TLP).

Superscalar means executing multiple instructions at the same time while chip-level multithreading (CMT) executes instructions from multiple threads within one processor chip at the same time. There are many ways to support more than one thread within a chip, namely:

• Interleaved multithreading: Interleaved issue of multiple instructions from different threads, also referred to as temporal multithreading. It can be further divided into fine-grain multithreading or coarse-grain multithreading depending on the frequency of interleaved issues. Fine-grain multithreading—such as in a barrel processor—issues instructions for different threads after every cycle, while coarse-grain multithreading only switches to issue instructions from another thread when the current executing thread causes some long latency events (like page fault etc.). Coarse-grain multithreading is more common for less context switch between threads. For example, Intel's Montecito processor uses coarse-grain multithreading, while Sun's UltraSPARC T1 uses fine-grain multithreading. For those processors that have only one pipeline per core, interleaved multithreading is the only possible way, because it can issue at most one instruction per cycle.
• Simultaneous multithreading (SMT): Issue multiple instructions from multiple threads in one cycle. The processor must be superscalar to do so.
• Chip-level multiprocessing (CMP or multicore): integrates two or more processors into one chip, each executing threads independently.
• Any combination of multithreaded/SMT/CMP.

The key factor to distinguish them is to look at how many instructions the processor can issue in one cycle and how many threads from which the instructions come. For example, Sun Microsystems' UltraSPARC T1 (known as "Niagara" until its November 14, 2005 release) is a multicore processor combined with fine-grain multithreading technique instead of simultaneous multithreading because each core can only issue one instruction at a time.