An interprocessor memory access arbitrating scheme for the S-3800 vector supercomputer

Reports an instruction-based variable priority scheme which achieves high sustained memory throughput on a tightly coupled multiprocessor (TCMP) vector supercomputer. We analyze the two types of priority control for arbitrating interprocessor memory access conflict. In the case of request level priority control, mutual obstruction causes performance degradation, while in the case of fixed priority control, it is caused by memory bank occupation. Mutual obstruction is caused by requests of different instructions that interfere with each other, and memory bank occupation is caused by continuous accessing of the same memory bank by higher priority instructions. The instruction-based variable priority scheme works as follows: (1) the priority of each pipeline is usually changed at the end of an instruction. (2) The priority is changed more than once in the middle of an instruction, such as a stride multiple-of-8 or indirect access instruction which may occupy the same memory bank by itself. This strategy reduces mutual obstruction because the priority of each pipeline is stable in the middle of an instruction. It also reduces memory bank occupation because opportunity for memory access among different instructions is made equal by changing the priority at the end of on instruction. Moreover, it prevents memory bank occupation by stride multiple-of-8 or indirect access instruction, by changing the priority more frequently. Consequently, high sustained memory throughput can be achieved on TCMP vector supercomputers. We implemented this scheme in Hitachi´s S-3800 supercomputer