SOUP-N-SALAD: Allocation-Oblivious Access Latency Reduction with Asymmetric DRAM Microarchitectures

Memory access latency has a significant impact on application performance. Unfortunately, the random access latency of DRAM has been scaling relatively slowly, and often directly affects the critical path of execution, especially for applications with insufficient locality or memory-level parallelism. The existing low-latency DRAM organizations either incur significant area overhead or burden the software stack with non-uniform access latency. This paper proposes two microarchitectural techniques to provide uniformly low access time over the entire DRAM chip. The first technique is SALAD, a new DRAM device architecture that provides symmetric access latency with asymmetric DRAM bank organizations. Because local regions have lower data transfer time due to their proximity to the I/O pads, SALAD applies high aspect-ratio (i.e., low-latency) mats only to remote regions to offset the difference in data transfer time, resulting in symmetrically low latency across regions. The second technique is SOUP (skewed organization of µ banks with pipelined accesses), which leverages asymmetry in column access latency within a region due to non-uniform distance to the column decoders. By starting I/O transfers as soon as data from near cells arrive, instead of waiting for the entire column data, SOUP further saves two memory clock cycles for column accesses for all regions. The resulting design, called SOUP-N-SALAD, improves IPC and EDP by 9.6% (11.2%) and 18.2% (21.8%) over the baseline DDR4 device, respectively, for memory-intensive SPEC CPU2006 workloads without any software modifications, while incurring only 3% (6%) area overhead.