Necromancer: enhancing system throughput by animating dead cores

Aggressive technology scaling into the nanometer regime has led to a host of reliability challenges in the last several years. Unlike on-chip caches, which can be efficiently protected using conventional schemes, the general core area is less homogeneous and structured, making tolerating defects a much more challenging problem. Due to the lack of effective solutions, disabling non-functional cores is a common practice in industry to enhance manufacturing yield, which results in a significant reduction in system throughput. Although a faulty core cannot be trusted to correctly execute programs, we observe in this work that for most defects, when starting from a valid architectural state, execution traces on a defective core actually coarsely resemble those of fault-free executions. In light of this insight, we propose a robust and heterogeneous core coupling execution scheme, Necromancer, that exploits a functionally dead core to improve system throughput by supplying hints regarding high-level program behavior. We partition the cores in a conventional CMP system into multiple groups in which each group shares a lightweight core that can be substantially accelerated using these execution hints from a potentially dead core. To prevent this undead core from wandering too far from the correct path of execution, we dynamically resynchronize architectural state with the lightweight core. For a 4-core CMP system, on average, our approach enables the coupled core to achieve 78.5% of the performance of a fully functioning core. This defect tolerance and throughput enhancement comes at modest area and power overheads of 5.3% and 8.5%, respectively.