Quality-aware data allocation in approximate DRAM?

Approximate computing is an emerging design paradigm that leverages the inherent error tolerance present in many applications to optimize their power consumption and performance. Due to the forgiving nature of these error-resilient applications, highly precise input data is not always necessary for them to produce outputs of acceptable quality. This makes memory, the place where data is stored, a suitable component for introducing errors or approximations in return for considerable energy savings. Towards this end, this paper proposes, for the first time, a systematic way for constructing a quality-aware approximate DRAM system. Our design is based upon an extensive experimental characterization of memory errors as a function of the DRAM refresh rate. Leveraging the insights gathered from this characterization, we propose four novel strategies for partitioning the DRAM into a number of quality bins based on the frequency, location, and nature of bit errors in each of the physical pages. During allocation, critical data is placed in the highest quality bin containing only accurate pages and approximate data is allocated to bins sorted in descending order of quality. We validate our proposed scheme on several error-resilient applications implemented using an Altera Stratix IV GX FPGA based Terasic TR4-230 development board containing a 1GB DDR3 DRAM module. Experimental results demonstrate a significant improvement in the energy-quality trade-off compared to previous work and show a reduction in DRAM refresh power of up to 73% with minimal loss in output quality.