Ganger

University of Michigan Technical Report CSE-TR-243-95, June, 1995.

Gregory R. Ganger 

Department of Electrical Engineering and Computer Science
The University of Michigan, Ann Arbor 48109-2122

This dissertation demonstrates that the conventional approach for evaluating the perfor­ mance of an I/O subsystem design, which is based on standalone subsystem models, is too narrow in scope. In particular, conventional methodology treats all I/O requests equally, ignoring differences in how individual request response times affect system behavior. As a result, it often leads to inaccurate performance predictions and can thereby lead to incor­ rect conclusions and poor design choices. A new methodology, which expands the model's scope to include other important system components (e.g., CPUs and system software), is proposed and shown to enable accurate predictions of both subsystem and overall system performance. This dissertation focuses on two specific problems with conventional methodology: 1. Benchmark workloads are often not representative of reality in that they do not accurately reflect feedback effects between I/O subsystem performance (in particular, individual request completion times) and the workload of requests (in particular, sub­ sequent request arrivals). 2. Changes in I/O subsystem performance (e.g., as measured by mean request response times) do not always translate into similar changes in overall system performance (e.g., as measured by mean elapsed times for user tasks). These problems are fundamental to the subsystem­oriented approach and are independent of the model's accuracy. The first problem is illustrated with several examples where commonly­ utilized workload generators trivialize feedback effects and produce unrealistic workloads. In each case, quantitative and/or qualitative errors result. The second problem is illustrated with a disk scheduling algorithm that gives priority to those requests that are most critical to overall system performance. This change increases overall system performance while decreasing storage subsystem performance (as indicated by subsystem metrics). In all of the experiments, the new methodology is shown to avoid the short­comings of conventional methodology.

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