SpongeDirectory: Flexible sparse directories utilizing multi-level memristors

Cache-coherent shared memory is critical for programmability in many-core systems. Several directory-based schemes have been proposed, but dynamic, non-uniform sharing make efficient directory storage challenging, with each giving up storage space, performance or energy. We introduce SpongeDirectory, a sparse directory structure that exploits multi-level memristory technology. SpongeDirectory expands directory storage in-place when needed by increasing the number of bits stored on a single memristor device, trading latency and energy for storage. We explore several SpongeDirectory configurations, finding that a provisioning rate of 0.5× with memristors optimized for low energy consumption is the most competitive. This optimal SpongeDirectory configuration has performance comparable to a conventional sparse directory, requires 18× less storage space, and consumes 8× less energy.