Extending Multicore Architectures to Exploit Hybrid Parallelism in Single-thread Applications

Chip multiprocessors with multiple simpler cores are gaining popularity because they have the potential to drive future performance gains without exacerbating the problems of power dissipation and complexity. Current chip multiprocessors increase throughput by utilizing multiple cores to perform computation in parallel. These designs provide real benefits for server-class applications that are explicitly multi-threaded. However, for desktop and other systems where single-thread applications dominate, multicore systems have yet to offer much benefit. Chip multiprocessors are most efficient at executing coarse-grain threads that have little communication. However, general-purpose applications do not provide many opportunities for identifying such threads, due to frequent use of pointers, recursive data structures, if-then-else branches, small function bodies, and loops with small trip counts. To attack this mismatch, this paper proposes a multicore architecture, referred to as Voltron, that extends traditional multicore systems in two ways. First, it provides a dual-mode scalar operand network to enable efficient inter-core communication and lightweight synchro synchronization. Second, Voltron can organize the cores for execution in either coupled or decoupled mode. In coupled mode, the cores execute multiple instruction streams in lock-step to collectively function as a wide-issue VLIW. In decoupled mode, the cores execute a set of fine-grain communicating threads extracted by the compiler. This paper describes the Voltron architecture and associated compiler support for orchestrating bi-modal execution.