UpDown: A Novel Architecture for Unlimited Memory Parallelism

The emergence of HBM as a high-volume memory product has made memory bandwidths of 1.2TB/s (1 stack) to 4.8TB/s (4 stacks) feasible. Exploiting such bandwidths requires high memory level parallelism, but the memory access mechanisms in today’s CPUs are ill-suited. We define the Memory Parallelism Abstract Machine (MPAM) that characterizes limits of a variety of various commercial and research designs. We propose the UpDown architecture that generates unlimited, cost-efficient memory parallelism using split-transaction accesses and a large compute-namespace to synchronize memory responses. Using MPAM, we show that UpDown can generate unlimited memory parallelism constrained only by the memory technology servicing the system and memory reference issue rate. Our evaluation shows that the smallest compute element of UpDown, a single lane can generate up to 3.5x more memory parallelism compared to a modern out-of-order CPU core, despite its much smaller area (<1%). We also show that 64 lanes of the UpDown architecture can sustain 1,673 outstanding memory references to nearly saturate the full bandwidth of 1 HBM3e stack (1.2TB/s). Finally, we also show that UpDown is much more energy and power efficient.