Modeling and measurement of the impact of input/output on system performance

The input/otrtput (1/0) subsystem is often the bottleneck in high performance computer systems where the CPU/Memory technology has been pushed to the limit. But recent microprocessor and workstation speeds ae beginning to shift the system balance to the point that 1/0 is becoming the bottleneck even in mid-range and low-end systems. In this work the 1/0 subsystem’s impact on system performartce is shown by modeling the relative performance of VAX uniprocessors with and without enhancement in the 1/0 subsystem. Tradhional system performance models were enhanced to include the effect of the I/O subsystem. The parameters modeling the I/O subsystem’s effect were identified as D[,O (the number of I/O bytes transferred per instruction executed by the CPU), trl,O (the &ansfer time per I/O byte), and Wq (the waiting time in the I/O subsystem). These parameters were measured on a VAX1 1/780 system by using special purpose hardware and were used to calibrate the enhanced system performance model. It is interesting to note that these measurements indicate that contemporary systems require a factor of eight increase over the I/O bandwidth requirement stated by the Amdhal-Case rule.