Jungwhan Choi

NUAT: A non-uniform access time memory controller

With rapid development of micro-processors, off-chip memory access becomes a system bottleneck. DRAM, a main memory in most computers, has concentrated only on capacity and bandwidth for decades to achieve high performance computing. However, DRAM access latency should also be considered to keep the development trend in multi-core era. Therefore, we propose NUAT which is a new memory controller focusing on reducing memory access latency without any modification of the existing DRAM structure. We only exploit DRAMs intrinsic phenomenon: electric charge variation in DRAM cell capacitors. Given the cost-sensitive DRAM market, it is a big advantage in terms of actual implementation. NUAT gives a score to every memory access request and the request with the highest score obtains a priority. For scoring, we introduce two new concepts: Partitioned Bank Rotation (PBR) and PBR Page Mode (PPM). First, PBR is a mechanism that draws information of access speed from refresh timing and position; the request which has faster access speed gains higher score. Second, PPM selects a better page mode between open- and close-page modes based on the information from PBR. Evaluations show that NUAT decreases memory access latency significantly for various environments.

Multiple clone row DRAM: a low latency and area optimized DRAM

Several previous works have changed DRAM bank structure to reduce memory access latency and have shown performance improvement. However, changes in the area-optimized DRAM bank can incur large area-overhead. To solve this problem, we propose Multiple Clone Row DRAM (MCR-DRAM), which uses existing DRAM bank structure without any modification.

Q-DRAM: Quick-Access DRAM with Decoupled Restoring from Row-Activation

The relatively high latency of DRAM is mostly caused by the long row-activation time which in fact consists of sensing and restoring time. Memory controllers cannot distinguish between them since they are performed consecutively by a single row-activation command. If these two steps are separated, the restoring can be delayed until DRAM access is uncongested. Hence, we propose Quick-Access DRAM (Q-DRAM) which discriminates between sensing and restoring. Our approach is to allow destructive access (i.e., only sensing is performed without restoring by a row-activation command) using per-bank multiple row-buffers. We call the destructive access and per-bank multiple row-buffers quick-access and quick-buffers (q-buffers) respectively. In addition, we propose Quick-access Trigger (Q-TRIGGER) and RESTORER to utilize Q-DRAM. Q-TRIGGER makes a decision whether quick-access is required or not, and RESTORER decides when to restore the data at the destructed cell. Specifically, RESTORER detects the proper timing to hide restoring time by predicting data bus occupation and by exploiting bank-level locality. Evaluations show that Q-DRAM significantly improved performance for both single- and multi-core systems.

Energy efficient data encoding in DRAM channels exploiting data value similarity

As DRAM data bandwidth increases, tremendous energy is dissipated in the DRAM data bus. To reduce the energy consumed in the data bus, DRAM interfaces with symmetric termination, such as Pseudo Open Drain (POD) and Low Voltage Swing Terminated Logic (LVSTL), have been adopted in modern DRAMs. In interfaces using asymmetric termination, the amount of termination energy is proportional to the hamming weight of the data words. In this work, we propose Bitwise Difference Encoding (BD-Encoding), which decreases the hamming weight of data words, leading to a reduction in energy consumption in the modern DRAM data bus. Since smaller hamming weight of the data words also reduces switching activity, switching energy and power noise are also both reduced. BD-Encoding exploits the similarity in data words in the DRAM data bus. We observed that similar data words (i.e. data words whose hamming distance is small) are highly likely to be sent over at similar times. Based on this observation, BD-coder stores the data recently sent over in both the memory controller and DRAMs. Then, BD-coder transfers the bitwise difference between the current data and the most similar data. In an evaluation using SPEC 2006, BD-Encoding using 64 recent data reduced termination energy by 58.3% and switching energy by 45.3%. In addition, 55% of the LdI/dt noise was decreased with BD-Encoding.

DRAM-Latency Optimization Inspired by Relationship between Row-Access Time and Refresh Timing

It is widely known that relatively long DRAM latency forms a bottleneck in computing systems. However, DRAM vendors are strongly reluctant to decrease DRAM latency due to the additional manufacturing cost. Therefore, we set our goal to reduce DRAM latency without any modification in the existing DRAM structure. To accomplish our goal, we focus on an intrinsic phenomenon in DRAM: electric charge variation in DRAM cell capacitors. Then, we draw two key insights: i) DRAM row-access latency of a row is a function of the elapsed time from when the row was last refreshed, and ii) DRAM row-access latency of a row is also a function of the remaining time until the row is next refreshed. Based on these two insights, we propose two mechanisms to reduce DRAM latency: NUAT-1 and NUAT-2. NUAT-1 exploits the first key insight and NUAT-2 exploits the second key insight. For evaluation, circuit- and system-level simulations are performed, which show the performance improvement for various environments.

In-DRAM Data Initialization

Initializing memory with zero data is essential for safe memory management. However, initializing a large memory area slows down the system significantly. The most likely cause for initialization to slow down the system is the limited DRAM initialization method. At present, the only way to initialize DRAM area is to execute multiple WRITE commands. However, the WRITE command slows the initialization because of its small granularity and data bus occupancy. In this brief, we propose an efficient in-DRAM initialization method inspired by the internal structure and operation of DRAM. The proposed method, called row reset, uses a DRAM row buffer to zero out a single DRAM row at a time. Row Reset allows for parallel initialization on multiple DRAM banks without using off-chip data transfer, thus reducing initialization time by up to 63 times. Row reset is a practical approach, because it can be implemented with existing circuitry in DRAM without additional area overhead.

Rank-Level Parallelism in DRAM

DRAM systems are hierarchically organized: Channel-Rank-Bank. A channel is connected to multiple ranks, and each rank has multiple banks. This hierarchical structure facilitates creating parallelisms in DRAM. The current DRAM architecture supports bank-level parallelism; as many rows as banks can be moved simultaneously at bank-level. However, rank-level parallelism is not supported. For this reason, only one column can be accessed at a time, although each rank has its own data bus that can carry a column. Namely, current DRAM operations do not exploit the structural opportunity created by multiple ranks. We, therefore, propose a novel DRAM architecture supporting rank-level parallelism. Thereby, as many columns as ranks can be moved concurrently at rank-level. In this paper, we illustrate the rank-level parallelism and its benefit in DRAM operations.

Bank-Group Level Parallelism

DDR4 SDRAM introduced a new hierarchy in DRAM organization: bank-group (BG). The main purpose of BG is to increase I/O bandwidth without growing DRAM-internal bus-width. We, however, found that other benefits can be derived from the new hierarchy. To achieve the benefits, we propose a new DRAM architecture using the BG-hierarchy, leading to a creation of BG-Level Parallelism (BGLP). By exploiting BGLP, the overall parallelism grows in DRAM operations. We also argue that BGLP is a feasible solution in the cost-sensitive DRAM industry because the additional cost is negligible and only cost-insensitive area needs to be modified.

Refresh-Aware Write Recovery Memory Controller

Current computer systems require large memory capacities to manage the tremendous volume of datasets. A DRAM cell consists of a transistor and a capacitor, and their size has a direct impact on DRAM density. While technology scaling can provide higher density, this benefit comes at the expense of low drivability, due to the increase in series resistance of the smaller transistor, which slows the process of restoring the charge in cells. DRAM operations require recovery processes due to the destructive nature of DRAM cells. Among such operations, the write recovery process has the most difficulty in meeting the timing constraints. In this paper, we explore an intrinsic mechanism in the DRAM write operation, and find a relation between restoration and retention times. Based on our observation, we propose a practical mechanism, Relaxed Refresh with Compensated Write Recovery (RRCW), which efficiently mitigates refresh overheads by providing longer restoration periods. Furthermore, to minimize the penalty of the longer restoration, we also introduce another mechanism, Refresh-Aware Write Recovery (RAWR), which appropriately curtails longer recovery time according to the waiting time until being refreshed. Lastly, we introduce a scheduling policy to efficiently utilize RAWR. Evaluations show that the benefits of our mechanisms increase as memory intensity increases.