As energy proportional computing has extended the success of DVFS (Dynamic
voltage and frequency scaling) to the entire system, DVFS control algorithms
will play a key role in reducing server clusters' power consumption. The focus
of this paper is to provide accurate cluster-level DVFS control for power
saving in a server cluster. To achieve this goal, we propose a request tracing
approach that online classifies the major causal path patterns and monitors
their performance data as a guide for accurate DVFS control.