DUO: Exposing On-Chip Redundancy to Rank-Level ECC for High Reliability

DRAM row and column sparing cannot efficiently tolerate the increasing inherent fault rate caused by continued process scaling. In-DRAM ECC (IECC), an appealing alternative to sparing, can resolve inherent faults without significant changes to DRAM, but it is inefficient for highly-reliable systems where rank-level ECC (RECC) is already used against operational faults. In addition, DRAM design in the near future (possibly as early as DDR5) may transfer data in longer bursts, which complicates high-reliability RECC due to fewer devices being used per rank and increased fault granularity. We propose dual use of on-chip redundancy (DUO), a mech- anism that bypasses the IECC module and transfers on-chip redundancy to be used directly for RECC. Due to its increased redundancy budget, DUO enables a strong and novel RECC for highly-reliable systems, called DUO SDDC. The long codewords of DUO SDDC provide fundamentally higher detection and correction capabilities, and several novel secondary-correction techniques integrate together to further expand its correction capability. According to our evaluation results, DUO shows performance degradation on par with or better than IECC (average 2–3%), while consuming less DRAM energy than IECC (average 4–14% overheads). DUO provides higher reliability than either IECC or the state-of-the-art ECC technique. We show the robust reliability of DUO SDDC by comparing it to other ECC schemes using two different inherent fault-error models.