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To date dynamic voltage/frequency scaling (DVFS) has been one of the most successful power-reduction techniques. However, ever-increasing process variability reduces the reliability of static random access memory (SRAM) at low voltages. This limits voltage scaling to a minimum operating voltage (VDDMIN). Larger SRAM cells, that are less sensitive to process variability, allow the use of lower VDDMIN. However, large-scale memory structures, e.g., the last-level cache (LLC) (that often determines the VDDMIN of the processor), cannot afford to use such large SRAM cells due to the die area constraint. In this paper we propose low-voltage LLC architectures that exploit 1) the DVFS characteristics of workloads running on high-performance processors, 2) the trade-off between SRAM cell size and VDDMIN, and 3) the fact that at lower voltage/frequency operating states the negative performance impact of having a smaller LLC capacity is reduced. Our proposed LLC architectures provide the same maximum performance and VDDMIN as the conventional architecture, while reducing the total LLC cell area by 15%-19% with negligible average runtime increase.
Date of Conference: 12-16 Feb. 2011