Techniques to Save Energy in Heterogeneous Multicore Architectures under QoS Constraints

Sammanfattning: Typically, applications are run with available system resources leading to over-provisioning of resources which can lead to high energy consumption. If the computational demand is specified, in terms of a Quality of Service (QoS) contract, it is possible to devote just enough resources to applications and thereby reduce energy consumption. Modern heterogeneous multicore platforms, such as ARM big.LITTLE, typically provide a multidimensional space of resources, called configuration space, such as Voltage-Frequency (V-F) settings, thread count and processor types, which can be configured at run-time to open up new opportunities for resource management. This thesis presents techniques to improve energy efficiency under the constraint of QoS by managing the resource allocation at run-time for applications run on heterogeneous multicore platforms. The applications considered are iterative with a computational deadline associated with each loop iteration. The proposed techniques apply to a framework that uses applications’ outer loop iterations as a means for progress-tracking and prediction of the execution time. A first contribution of the thesis is a resource management technique for single-threaded applications that uses core type (e.g. big or little cores) and V-F settings as a configuration space to select a configuration, for each iteration, based on the execution-time prediction of future iterations and computational deadlines. The thesis shows that an energy saving of 25% over the race-to-idle state-of-the-art technique is achieved without missing any deadlines. This scheme incurs only 0.6% and 0.8% of timing and energy overheads, respectively. A second contribution of the thesis is a novel resource-management policy for multi-threaded applications. Here, the configuration space is extended to also consider the thread count, i.e., the number of cores assigned to multi-threaded applications. The proposed technique first chooses the most energy-efficient configuration that meets the computational deadline. Since an iteration typically finishes before the deadline, the proposed technique collects the generated execution-time slack over subsequent iterations with the goal of selecting a configuration that can save more energy. To allow for on-line exploration of the configuration space, at low overhead, a third contribution of the thesis is an online, low-overhead prediction method based on interpolation, that measures the execution statistics at end points of each configuration-space dimension and interpolates the values at intermediate configurations. Overall, the proposed technique saves 61% energy compared to the state-of-the-art race-to-idle technique without missing any deadlines. Further, it only incurs 0.6% and 0.7% of timing and energy overheads, respectively.