Program balance and its impact on high performance RISC architectures

Information on the behavior of programs is essential for deciding the number and nature of functional units in high performance architectures. In this paper, we present studies on the balance of access and computation tasks on a typical RISC architecture, the MIPS. The MIPS programs are analyzed to find the demands they place on the memory system and the floating point or integer computation units. A balance metric that indicates the match of accessing power to computation power is calculated. It is observed that many of the SPEC floating point programs and kernels from supercomputing applications typically considered as computation intensive programs, place extensive demands on the memory system in terms of memory bandwidth. Access related instructions are seen to dominate most instruction streams. We discuss how these instruction stream characteristics can limit the instruction issue in superscalar processors. The properties of the dynamic instruction mix are used to alert computer architects to the importance of memory bandwidth. Single instruction stream parallelism will not be much greater than two if memory bandwidth is only one. A decoupled access/execute architecture with multiple load/store units and queues which alleviate the balance problem is presented.<
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