Understanding fundamental design choices in single-ISA heterogeneous multicore architectures

Abstract

Single-ISA heterogeneous multicore processors have gained substantial interest over the past few years because of their power efficiency, as they offer the potential for high overall chip throughput within a given power budget. Prior work in heterogeneous architectures has mainly focused on how heterogeneity can improve overall system throughput. To what extent heterogeneity affects per-program performance has remained largely unanswered. In this article, we aim at understanding how heterogeneity affects both chip throughput and per-program performance; how heterogeneous architectures compare to homogeneous architectures under both performance metrics; and how fundamental design choices, such as core type, cache size, and off-chip bandwidth, affect performance. We use analytical modeling to explore a large space of single-ISA heterogeneous architectures. The analytical model has linear-time complexity in the number of core types and programs of interest, and offers a unique opportunity for exploring the large space of both homogeneous and heterogeneous multicore processors in limited time. Our analysis provides several interesting insights: While it is true that heterogeneity can improve system throughput, it fundamentally trades per-program performance for chip throughput; although some heterogeneous configurations yield better throughput and per-program performance than homogeneous designs, some homogeneous configurations are optimal for particular throughput versus per-program performance trade-offs. Two core types provide most of the benefits from heterogeneity and a larger number of core types does not contribute much; job-to-core mapping is both important and challenging for heterogeneous multicore processors to achieve optimum performance. Limited off-chip bandwidth does alter some of the fundamental design choices in heterogeneous multicore architectures, such as the need for large on-chip caches for achieving high throughput, and per-program performance degrading more relative to throughput under constrained off-chip bandwidth.