Manifestation of Precharge Faults in High Speed DRAM Devices

Abstract: High speed DRAMs today suffer from an increased sensitivity to interference and noise problems. Signal integrity issues, caused by bit line and word line coupling, result in their own set of faults, and increase the complexity of already known faults. This paper describes the influence of bit line coupling on precharge faults, where the memory is rendered unable to set the proper precharge voltages at the end of each operation, which causes the memory to fail in subsequent read operations. This kind of bit line coupling effect on precharge behavior has been observed in high speed DRAMs at Qimonda. This paper gives a detailed analysis of the problem, and suggests effective tests to detect it. The paper also describes the results of an industrial test evaluation on actual DRAMs chips, performed to validate the effectiveness of the proposed tests

Understanding and Improving the Latency of DRAM-Based Memory Systems

Over the past two decades, the storage capacity and access bandwidth of main memory have improved tremendously, by 128x and 20x, respectively. These improvements are mainly due to the continuous technology scaling of DRAM (dynamic random-access memory), which has been used as the physical substrate for main memory. In stark contrast with capacity and bandwidth, DRAM latency has remained almost constant, reducing by only 1.3x in the same time frame. Therefore, long DRAM latency continues to be a critical performance bottleneck in modern systems. Increasing core counts, and the emergence of increasingly more data-intensive and latency-critical applications further stress the importance of providing low-latency memory access. In this dissertation, we identify three main problems that contribute significantly to long latency of DRAM accesses. To address these problems, we present a series of new techniques. Our new techniques significantly improve both system performance and energy efficiency. We also examine the critical relationship between supply voltage and latency in modern DRAM chips and develop new mechanisms that exploit this voltage-latency trade-off to improve energy efficiency. The key conclusion of this dissertation is that augmenting DRAM architecture with simple and low-cost features, and developing a better understanding of manufactured DRAM chips together lead to significant memory latency reduction as well as energy efficiency improvement. We hope and believe that the proposed architectural techniques and the detailed experimental data and observations on real commodity DRAM chips presented in this dissertation will enable development of other new mechanisms to improve the performance, energy efficiency, or reliability of future memory systems.Comment: PhD Dissertatio