Sectored DRAM: An Energy-Efficient High-Throughput and Practical Fine-Grained DRAM Architecture

There are two major sources of inefficiency in computing systems that use modern DRAM devices as main memory. First, due to coarse-grained data transfers (size of a cache block, usually 64B between the DRAM and the memory controller, systems waste energy on transferring data that is not used. Second, due to coarse-grained DRAM row activation, systems waste energy by activating DRAM cells that are unused in many workloads where spatial locality is lower than the large row size (usually 8-16KB). We propose Sectored DRAM, a new, low-overhead DRAM substrate that alleviates the two inefficiencies, by enabling fine-grained DRAM access and activation. To efficiently retrieve only the useful data from DRAM, Sectored DRAM exploits the observation that many cache blocks are not fully utilized in many workloads due to poor spatial locality. Sectored DRAM predicts the words in a cache block that will likely be accessed during the cache block's cache residency and: (i) transfers only the predicted words on the memory channel, as opposed to transferring the entire cache block, by dynamically tailoring the DRAM data transfer size for the workload and (ii) activates a smaller set of cells that contain the predicted words, as opposed to activating the entire DRAM row, by carefully operating physically isolated portions of DRAM rows (MATs). Compared to prior work in fine-grained DRAM, Sectored DRAM greatly reduces DRAM energy consumption, does not reduce DRAM throughput, and can be implemented with low hardware cost. We evaluate Sectored DRAM using 41 workloads from widely-used benchmark suites. Sectored DRAM reduces the DRAM energy consumption of highly-memory-intensive workloads by up to (on average) 33% (20%) while improving their performance by 17% on average. Sectored DRAM's DRAM energy savings, combined with its system performance improvement, allows system-wide energy savings of up to 23%.