Smart Refresh: An Enhanced Memory Controller Design for Reducing Energy in Conventional and 3D Die-Stacked DRAMs

DRAMs require periodic refresh for preserving data stored in them. The refresh interval for DRAMs depends on the vendor and the design technology they use. For each refresh in a DRAM row, the stored information in each cell is read out and then written back to itself as each DRAM bit read is self-destructive. The refresh process is inevitable for maintaining data correctness, unfortunately, at the expense of power and bandwidth overhead. The future trend to integrate layers of 3D die-stacked DRAMs on top of a processor further exacerbates the situation as accesses to these DRAMs will be more frequent and hiding refresh cycles in the available slack becomes increasingly difficult. Moreover, due to the implication of temperature increase, the refresh interval of 3D die-stacked DRAMs will become shorter than those of conventional ones. This paper proposes an innovative scheme to alleviate the energy consumed in DRAMs. By employing a time-out counter for each memory row of a DRAM module, all the unnecessary periodic refresh operations can be eliminated. The basic concept behind our scheme is that a DRAM row that was recently read or written to by the processor (or other devices that share the same DRAM) does not need to be refreshed again by the periodic refresh operation, thereby eliminating excessive refreshes and the energy dissipated. Based on this concept, we propose a low-cost technique in the memory controller for DRAM power reduction. The simulation results show that our technique can reduce up to 86% of all refresh operations and 59.3% on the average for a 2 GB DRAM. This in turn results in a 52.6% energy savings for refresh operations. The overall energy saving in the DRAM is up to 25.7% with an average of 12.13% obtained for SPLASH-2, SPECint2000, and Biobench benchmark programs simulated on a 2 GB DRAM. For a 64 MB 3D DRAM, the energy saving is up to 21% and 9.37% on an average when the refresh rate is 64 ms. For a faster 32 ms refresh rate the maximum and average savings are 12% and 6.8% respectively.