  Modern client processors typically use one of three commonly-used power
delivery network (PDN): 1) motherboard voltage regulators (MBVR), 2) integrated
voltage regulators (IVR), and 3) low dropout voltage regulators (LDO). We
observe that the energy-efficiency of each of these PDNs varies with the
processor power (e.g., thermal design power (TDP) and dynamic power-state) and
workload characteristics. This leads to energy inefficiency and performance
loss, as modern client processors operate across a wide spectrum of power
consumption and execute a wide variety of workloads. We propose FlexWatts, a
hybrid adaptive PDN for modern client processors whose goal is to provide high
energy-efficiency across the processor's wide range of power consumption and
workloads by dynamically allocating PDNs to processor domains. FlexWatts is
based on three key ideas. First, it combines IVRs and LDOs in a novel way to
share multiple on-chip and off-chip resources. This hybrid PDN is allocated for
processor domains with a wide power consumption range and it dynamically
switches between two modes: IVR-Mode and LDO-Mode, depending on the power
consumption. Second, for all other processor domains, FlexWatts statically
allocates off-chip VRs. Third, FlexWatts introduces a prediction algorithm that
switches the hybrid PDN to the mode that is the most beneficial. To evaluate
the tradeoffs of PDNs, we develop and open-source PDNspot, the first validated
architectural PDN model that enables quantitative analysis of PDN metrics.
Using PDNspot, we evaluate FlexWatts on a wide variety of SPEC CPU2006,
3DMark06, and battery life workloads against IVR, the state-of-the-art PDN in
modern client processors. For a 4W TDP processor, FlexWatts improves the
average performance of the SPEC CPU2006 and 3DMark06 workloads by 22% and 25%,
respectively. FlexWatts has comparable cost and area overhead to IVR.
