EM2: A Scalable Shared-Memory Multicore Architecture

We introduce the Execution Migration Machine (EM2), a novel, scalable shared-memory architecture for large-scale multicores constrained by off-chip memory bandwidth. EM2 reduces cache miss rates, and consequently off-chip memory usage, by permitting only one copy of data to be stored anywhere in the system: when a thread wishes to access an address not locally cached on the core it is executing on, it migrates to the home core for that data and continues execution. Using detailed simulations of a 256-core chip multiprocessor on the SPLASH-2 benchmarks, we show that EM2 outperforms directory-based cache-coherence 1.13× on average using a high-bandwidth electrical network and 2× with an optical network. In addition, because of the dramatic reduction in off-chip memory accesses, EM2 improves energy consumption by 1.3× on average, and the energy-delay product by up to 5.4× over cache coherence.