Introduction

---

The hardware and software structure of large Web sites is increasingly complex, and the behavior characteristics of the workloads are at best poorly understood, or at worst, essentially unknown because the workload has yet to be implemented. Even with this growing complexity, typical IT infrastructures can be analyzed and related models developed to assist in predicting and planning how to meet future requirements. IBM developed the High-Volume Web Site (HVWS) Performance Simulator for WebSphere to estimate the performance of complex configurations. The simulator has these key features:

• Applications are defined based on the intended uses of the Web site using the workload patterns associated with high-volume sites. Detailed knowledge of the workload characteristics is not necessary, although it can be used to increase the accuracy of the simulation. The simulator workload library contains measurements of real customer applications for these typical workload patterns: shopping, banking, brokerage, auction, portal, B2B, reservation system, and inventory management system. There is an additional pattern, user-defined, where the user can enter the characteristics of their specific application supported either by measurement data or documented (and verifiable) assumptions.

• The simulator includes built-in performance characteristics of selected pSeries, xSeries, zSeries, and Sun models.

• The simulator displays performance results in sufficient detail to allow users to assess the adequacy of a given configuration for their requirements, and to provide insight into where the bottlenecks are likely to occur. This allows the simulator to be useful for planning capacity, evaluating infrastructure and workload changes, projecting Web site scalability, and reducing the cost of prototyping.

The HVWS Simulator supports performance modeling based on a generalized view of the infrastructure options shown in Figure 9-1. The simulator enables users to adjust the number of tiers to allow a more accurate model of the site configuration under consideration.

Figure 9-1: Web server topology

The HVWS Simulator estimates performance using an analytic model based on an enhanced version of the G/M/K queuing model. In this technique users are iteratively added to the system in an incremental fashion. Two complete sets of calculations are performed in the simulator. The base set of calculations uses built in (or user provided) measured data for CPU and disk I/O. A more conservative base plus contingency set of calculations is also performed by adding a user-supplied contingency factor to each of the measured data values before the calculation. Both results are then transformed to the 'target' configuration using built-in scaling coefficients taken from industry standard benchmarks and measurements.

At each step the calculated results are compared against the user-selected performance target(s) to determine if a target has been reached or if an early resource depletion (CPU or disk bandwidth) has occurred in any component of the infrastructure being evaluated. Resource depletion events signal the need for configuration adjustments and bring all calculations to a stop. Examination of the displayed provide the user with information about where the bottleneck occurred and what the aggregated load and performance indicators were.

---