Performability: An e-utility Imperative

J. F. Meyer and L. F. Spainhower


An e-business infrastructure is emerging to support a new generation of server-hosted, network-delivered applications and services: the e-utility. An e-utility is implemented by scalable clusters of small servers which, collectively, deliver Web content and services for multiple businesses. Demands for such services, as experienced by an entire utility, typically consist of concurrent and independent read-only requests, resulting in a randomly varying workload whose peak rate far exceeds the average request-rate. Hence, significant overprovisioning of capacity is currently quite common. This situation is undesirable for the utility provider and differential quality of service (QoS) is emerging as a possible remedy. Over time, the allocation of computing resources may change from one business to another, depending on prior contractual agreements (e.g.,workload prioritization, resource limits) and the availability of other servers (as randomly affected by faults, local workloads, etc.). Hence, dynamics of both an e-utility and its operational environment can substantially affect QoS perceived by users.

To evaluate e-utility QoS, we contend that the use of model-based performability evaluation techniques and tools is imperative. Our discussion begins with some relevant background regarding both e-utilities and the concept of performability. The above contention is then supported with a more general argument based on definitions of QoS provided by the telecom and Internet communities. Specifically, we show that performability measures specialize to QoS measures and, moreover, provide an effective means of expressing the "collective effect" of lower level measures. A performability approach to evaluating e-utility QoS is then discussed, where the main steps concern QoS measure specification, workload characterization, system model construction, measure formulation (in terms of model behavior), and finally measure evaluation.

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