Lightweight SIMT core designs for intelligent 3D stacked DRAM

In this work we present an analysis of the Harmonica stream multiprocessor, a light-weight, parameterized, open-source single-instruction-multiple-thread (SIMT) core designed for integration within 3D-stacked DRAM. We evaluate the range of Harmonica designs afforded by the architecture's parameter space in the role of a vault-level accelerator, augmenting a design similar to the Micron Hybrid Memory Cube into an array of compact accelerated DRAM channels. In this role, with a small SRAM cache, Harmonica cores are capable of providing the requisite small footprint, energy efficiency, latency tolerance, and bandwidth demand to perform well. The instruction set and microarchitecture of Harmonica are both novel, providing a lightweight interface for thread creation within the SIMT model and a simple design that issues a single warp per cycle, simplifying the register file design compared to high-performance GPUs, and providing parameters for attributes from the number of warps and threads per warp to the number of general purpose registers per thread. For our suite of analytics-oriented benchmarks, Harmonica cores consuming on the order of 100mW of power maintain a demand for an average of 12GB/s of bandwidth while tolerating the latency present in a DRAM-based memory system.