A Composable Worst Case Latency Analysis for Multi-Rank DRAM Devices under Open Row Policy

Abstract

As multi-core systems are becoming more popular in real-time embedded systems, strict timing requirements for accessing shared resources must be met. In particular, a detailed latency analysis for Double Data Rate Dynamic RAM (DDR DRAM) is highly desirable. Several researchers have proposed predictable memory controllers to provide guaranteed memory access latency. However, the performance of such controllers sharply decreases as DDR devices become faster and the width of memory buses is increased. High-performance Commercial-Off-The-Shelf (COTS) memory controllers in general-purpose systems employ open row policy to improve average case access latencies and memory throughput, but the use of such policy is not compatible with existing real-time controllers. In this article, we present a new memory controller design together with a novel, composable worst case analysis for DDR DRAM that provides improved latency bounds compared to existing works by explicitly modeling the DRAM state. In particular, our approach scales better with increasing memory speed by predictably taking advantage of shorter latency for access to open DRAM rows. Furthermore, it can be applied to multi-rank devices, which allow for increased access parallelism. We evaluate our approach based on worst case analysis bounds and simulation results, using both synthetic tasks and a set of realistic benchmarks. In particular, benchmark evaluations show up to 45% improvement in worst case task execution time compared to a competing predictable memory controller for a system with 16 requestors and one rank.