Algorithms and Parallel Computing

2.8 MULTITHREADED PROCESSOR

As defined at the start of this chapter, a thread is a portion of a program that shares processor resources with other threads. A multithreaded processor is a processor capable of running several software threads simultaneously. Of course, a simple processor has only one ALU and can manage to run one thread at a time. Ungerer et al. [24] provide a comprehensive discussion on threads. Figure 2.23a shows the case of a simple processor running an OS that allows running only one thread. We see in the figure the situation when the thread T₀ stalls, such as due to waiting for a memory access or a cache miss. Of course, the program execution halts until the memory access has been completed. Figure 2.23b shows the case of a single processor running an OS that supports multithreading. Two threads, T₀ and T₁, are available. The OS schedules T₀ for execution and when T₁ stalls, thread T₁ is immediately loaded and runs until T₀ is ready to resume. In such a case, a preemptive OS scheduler is assumed where the execution of T₁ is stopped when T₀ is ready to resume operation.

Figure 2.23 Multithreading in a single processor. (a) Single processor running a single thread. (b) Single processor running several threads. (c) Superscalar processor running a single thread. (d) Superscalar processor running multiple threads.

Figure 2.23c shows the case of a two-way superscalar ...