A Single-Tier Virtual Queuing Memory Controller Architecture for Heterogeneous MPSoCs

Abstract

Heterogeneous MPSoCs typically integrate diverse cores, including application CPUs, GPUs, and HD coders. These cores commonly share an off-chip memory to save cost and energy, but their memory accesses often interfere with each other, leading to undesirable consequences like a slowdown of application performance or a failure to sustain real-time performance. The memory controller plays a central role in meeting the QoS needs of real-time cores while maximizing CPU performance. Previous QoS-aware memory controllers are based on a classic two-tier queuing architecture that buffers memory transactions at the first tier, followed by a second tier that buffers translated DRAM commands. In these designs, QoS-aware policies are used to schedule competing transactions at the first stage, but the translated DRAM commands are served in FIFO order at the second stage. Unfortunately, once the scheduled transactions have been forwarded to the command stage, newly arriving transactions that may be more critical cannot be served ahead of those translated commands that are already queued at the second stage. To address this, we propose a scalable memory controller architecture based on single-tier virtual queuing (STVQ) that maintains a single tier of request queues and employs an efficacious scheduler that considers both QoS requirements and DRAM bank states. In comparison with previous QoS-aware memory controllers, the proposed STVQ memory controller reduces CPU slowdown by up to 13.9% while satisfying all frame rate requirements. We propose further optimizations that can significantly increase row-buffer hits by up to 66.2% and reduce memory latency by up to 19.8%.