Improving energy efficiency of DRAM by exploiting half page row access

DRAM energy is an important component to optimize in modern computing systems. One outstanding source of DRAM energy is the energy to fetch data stored on cells to the row buffer, which occurs during two DRAM operations, row activate and refresh. This work exploits previously proposed half page row access, modifying the wordline connections within a bank to halve the number of cells fetched to the row buffer, to save energy in both cases. To accomplish this, we first change the data wire connections in the sub-array to reduce the cost of row buffer overfetch in multi-core systems which yields a 12% energy savings on average and a slight performance improvement in quad-core systems. We also propose charge recycling refresh, which reuses charge left over from a prior half page refresh to refresh another half page. Our charge recycling scheme is capable of reducing both auto- and self-refresh energy, saving more than 15% of refresh energy at 85°C, and provides even shorter refresh cycle time. Finally, we propose a refresh scheduling scheme that can dynamically adjust the number of charge recycled half pages, which can save up to 30% of refresh energy at 85°C.