Exploiting Heterogeneous Multicore Processors Through Fine-Grained Scheduling and Low-Overhead Thread Migration

Heterogeneous (also known as asymmetric) multicore processors (HMPs) offer significant advantages over homogeneous multicores in terms of both power and performance. Power-efficient cores can be paired with higher-performance cores to achieve advantageous power/performance tradeoffs. Particular cores could also be tailored to efficiently meet the demands of particular application domains. Unfortunately, HMPs also create unique challenges in effective mapping of running processes to cores. The greater the diversity of cores, the more complex this problem becomes.Existing dynamic scheduling approaches for HMPs fall into two categories: sampling and prediction. Sampling approaches permute running applications to across core types to find the best-performing assignment. This sampling step hurts performance and power due to time spent migrating threads through non-optimal assignments. Alternatively, prediction-based approaches estimate the performance for each application on different types of cores and choose the schedule with the best estimated performance. Prediction eliminates the cost of sampling but may result in sub-optimal scheduling decisions.This dissertation introduces new and novel phase-identification-based online schedulers for HMPs that combine aspects of both sampling and prediction approaches by identifying phases of execution (instruction sequences with similar behavior), sampling new phases, recognizing repeating phases and reusing recorded phase information to predict the best performing schedule and optimize the schedule for either performance or power consumption. While previous approaches utilized only phase-change detection to begin evaluating new schedules, the proposed approaches recognize the current phase of each executing thread and reuse phase information recorded in a Signature History Table when the same or similar behavior of programs reoccurs. This dissertation further proposes machine-learning based schedulers that learns effective scheduling policies using the same characteristics of these program phases.Exploiting differences between relatively short duration phases using the presented scheduling techniques results in frequent thread migrations that can harm performance. Operating system (OS) context switching can be time consuming. To reduce this context switching overhead, a context switching circuit that both accelerates thread switches among cores in HMPs and reduces switching cost within each core (multitasking) is further introduced in this work. This novel context switch circuit enables low-overhead hardware-level thread migration between cores on a chip and results in up to1380X speedup as compared to an OS context switch. Together with the presented scheduling approaches, this mechanism enables efficient and fine-grained scheduling for HMPs