Directoryless shared memory architecture using thread migration and remote access

Abstract

Chip multiprocessors (CMPs) have become mainstream in recent years, and, for scalability reasons, high-core-count designs tend towards tiled CMPs with physically distributed caches. In order to support shared memory, current many-core CMPs maintain cache coherence using distributed directory protocols, which are extremely difficult and error-prone to implement and verify. Private caches with directory-based coherence also provide suboptimal performance when a thread accesses large amounts of data distributed across the chip: the data must be brought to the core where the thread is running, incurring delays and energy costs. Under this scenario, migrating a thread to data instead of the other way around can improve performance. In this thesis, we propose a directoryless approach where data can be accessed either via a round-trip remote access protocol or by migrating a thread to where data resides. While our hardware mechanism for fine-grained thread migration enables faster migration than previous proposals, its costs still make it crucial to use thread migrations judiciously for the performance of our proposed architecture. We, therefore, present an on-line algorithm which decides at the instruction level whether to perform a remote access or a thread migration. In addition, to further reduce migration costs, we extend our scheme to support partial context migration by predicting the necessary thread context. Finally, we provide the ASIC implementation details as well as RTL simulation results of the Execution Migration Machine (EM² ), a 110-core directoryless shared-memory processor.