Impact of Process Variations on Multicore Performance Symmetry

Multi-core architectures introduce a new granularity at which process variations may occur, yielding asymmetry among cores that were designed---and that software expects---to be symmetric in performance. The chief source of this phenomenon are highly correlated, "systematic" within-die variations such as optical imperfections yielding variations across the exposure field. Per-core voltages can be used to bring all cores to the same performance level, but this compensation strategy also affects power, chiefly due to leakage power. Boosting a core's frequency may therefore boost its leakage sufficiently to engage thermal throttling. This sets up a tradeoff between static performance asymmetry due to frequency variation versus dynamic performance asymmetry due to thermal throttling. This paper explores the potential magnitude of these effects.