中文摘要：

动态多层Web系统在运行时会受到许多不确定性因素的影响．同时，在不同的负载模式下具有不同的性能特性，需要不同的性能模型进行描述．为消除不确定性因素对系统性能的影响，基于反馈控制原理设计的性能保障机制主要采用单一、固定的系统性能模型，对动态Web系统变化的性能特征考虑不够．在负载呈波动且具有不可预测特性的Intemet环境中，这会降低性能目标的精确性和稳定性．采用自适应控制的思想，以满足请求平均响应时间为目标，提出了一种基于在线评估系统性能模型的保障机制，并采用两个不同类型的事务AWeb测试基准，对所提方法进行了评价．结果表明，该方法能够有效减轻变化负载模式下响应时间与预期目标的偏离程度．

英文摘要：

Dynamic multi-tiered Web system can be affected by many unexpected factors during runtime. At the same time, it has different performance characteristic under different workload pattern condition. Accordingly, different performance models are needed to describe the system. Currently, performance guarantee mechanism based on feedback control theory, which aims to eliminate the effect of unexpected factors, often employs single and fixed system performance model which can not adapt to changing performance characteristics. Under fluctuant and unexpected workload condition of Internet, the method can decrease the precision and stability of performance. To guarantee anticipated response time goal, this paper presents an adaptive performance guarantee mechanism based on online estimation of performance model. Using two complementary transactional Web benchmarks to evaluate the techniques experimentally shows that the mechanism can efficiently reduce the squared departure of performance goal under varying workload.