The Case of the Missing Supercomputer Performance: Achieving Optimal Performance on the 8,192 Processors of ASCI Q

In this paper we describe how we improved the effective performance of ASCI Q, the world\´s second-fastest supercomputer, to meet our expectations. Using an arsenal of performance-analysis techniques including analytical models, custom microbenchmarks, full applications, and simulators, we succeeded in observing a serious-but previously undetected-performance problem. We identified the source of the problem, eliminated the problem, and "closed the loop" by demonstrating up to a factor of 2 improvement in application performance. We present our methodology and provide insight into performance analysis that is immediately applicable to other large-scale supercomputers.