Improving Application Performance by Dynamically Balancing Speed and Complexity in a GALS Microprocessor

Microprocessors are traditionally designed to provide “best overall” performance across a wide range of applications and operating environments. Several groups have proposed hardware techniques that save energy by “downsizing” hardware resources that are underutilized by particular applications. We explore the converse: “upsizing” hardware resources in order to improve performance relative to an aggressively clocked baseline processor. Our proposal depends critically on the ability to change frequencies independently in separate domains of a globally asynchronous, locally synchronous (GALS) microprocessor. We use a variant of our multiple clock domain (MCD) processor, with four independently clocked domains. Each domain is streamlined with modest hardware structures for very high clock frequency. Key structures can then be upsized on demand to exploit more distant parallelism, improve branch prediction, or increase cache capacity. Although doing so requires decreasing the associated domain frequency, other domain frequencies are unaffected. Measuring across a broad suite of application benchmarks, we find that configuring just once per application increases performance by an average of 17.6% compared to the best fully synchronous design. When adapting to application phases, performance improves by over 20%.