Improving cache lifetime reliability at ultra-low voltages

Voltage scaling is one of the most effective mechanisms to reduce microprocessor power consumption. However, the increased severity of manufacturing-induced parameter variations at lower voltages limits voltage scaling to a minimum voltage, Vccmin, below which a processor cannot operate reliably. Memory cell failures in large memory structures (e.g., caches) typically determine the Vccmin for the whole processor. Memory failures can be persistent (i.e., failures at time zero which cause yield loss) or non-persistent (e.g., soft errors or erratic bit failures). Both types of failures increase as supply voltage decreases and both need to be addressed to achieve reliable operation at low voltages. In this paper, we propose a novel adaptive technique to improve cache lifetime reliability and enable low voltage operation. This technique, multi-bit segmented ECC (MS-ECC) addresses both persistent and non-persistent failures. Like previous work on mitigating persistent failures, MS-ECC trades off cache capacity for lower voltages. However, unlike previous schemes, MS-ECC does not rely on testing to identify and isolate defective bits, and therefore enables error tolerance for nonpersistent failures like erratic bits and soft errors at low voltages. Furthermore, MS-ECC's design can allow the operating system to adaptively change the cache size and ECC capability to adjust to system operating conditions. Compared to current designs with single-bit correction, the most aggressive implementation for MS-ECC enables a 30% reduction in supply voltage, reducing power by 71% and energy per instruction by 42%.