We present KnightShift, a server-level heterogeneous server architecture that introduces an active low power mode, through the addition of a tightly-coupled compute node called the Knight, enabling two  ...  We propose to tackle the lack of energy proportionality at low utilization using server-level heterogeneity.  ...  Acknowledgement We would like to thank the anonymous reviewers for their valuable comments. We also thank Sabyasachi Ghosh and Mark Redekopp for their early contributions which inspired this work.  ...