LeadOut: Composing low-overhead frequency-enhancing techniques for single-thread performance in configurable multicores

Despite the ubiquity of multicores, it is as important as ever to deliver high single-thread performance. An ap- pealing way to accomplish this is by shutting down the idle cores in the chip and running the busy, performance- critical core(s) at higher-than-nominal frequencies. To en- able such frequencies, two low-overhead approaches ei- ther boost voltage beyond nominal values, or pair cores in leader-checker configurations and let them run beyond safe frequency margins. We observe that, in a large multicore with varying num- bers of busy cores, individual application of either of these two techniques is suboptimal. Each alone is often unable to bring the multicore all the way to its power or tempera- ture envelopes due to limitations in supply voltage or error rate. Moreover, we show that the two techniques are com- plementary, and can be synergistically combined to unlock much higher levels of single-thread performance. Finally, we demonstrate a dynamic controller that optimizes the two techniques. Our data shows that, given a 16-core multi- core where half of the cores are already busy, an additional, performance-critical thread now attains 34% higher perfor- mance than before, while consuming 220% more power.