Understanding program behavior is at the foundation of computer architecture and program optimization. Many programs have wildly different behavior on even the largest of scales (that is, over the program's complete execution). During one part of the execution, a program can be completely memory bound; in another, it can repeatedly stall on branch mispredicts. Average statistics gathered about a program might not accurately picture where the real problems lie. This realization has ramifications for many architecture and compiler techniques, from how to best schedule threads on a multithreaded machine, to feedback-directed optimizations, power management, and the simulation and test of architectures. Taking advantage of time-varying behavior requires a set of automated analytic tools and hardware techniques that can discover similarities and changes in program behavior on the largest of time scales.The challenge in building such tools is that during a program's lifetime it can execute billions or trillions of instructions. How can high-level behavior be extracted from this sea of instructions?The reality is this: The way a program's execution changes over time is not totally random; in fact, it often falls into repeating behaviors, called phases. Automatically identifying this phase behavior is the goal of our research and key to unlocking many new optimizations. We define a phase as a set of intervals (or slices in time) within a program's execution that have similar behavior, regardless of temporal adjacency. Recent research has shown that it is indeed possible to accurately identify and predict these phases in program behavior to capture meaningful phase behavior. 1-8The key observation for phase recognition is that any program metric is a direct function of the way a program traverses the code during execution. We can find this phase behavior and classify it by examining only the ratios in which different regions of code are being executed over time. We can simply and quickly collect this information using basic block vector profiles for off-line classification 4,6 or through dynamic branch profiling for online classification. 7 In addition, accurately capturing phase behavior through the computation of a single metric, independent of the underlying architectural details, means that it is pos-

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