The efficacy of error mitigation techniques for DRAM retention failures: a comparative experimental study.

As DRAM cells continue to shrink, they become more susceptible to retention failures. DRAM cells that permanently exhibit short retention times are fairly easy to identify and repair through the use of memory tests and row and column redundancy. However, the retention time of many cells may vary over time due to a property called Variable Retention Time (VRT). Since these cells intermittently transition between failing and non-failing states, they are particularly difficult to identify through memory tests alone. In addition, the high temperature packaging process may aggravate this problem as the susceptibility of cells to VRT increases after the assembly of DRAM chips. A promising alternative to manufacture-time testing is to detect and mitigate retention failures after the system has become operational. Such a system would require mechanisms to detect and mitigate retention failures in the field, but would be responsive to retention failures introduced after system assembly and could dramatically reduce the cost of testing, enabling much longer tests than are practical with manufacturer testing equipment. In this paper, we analyze the efficacy of three common error mitigation techniques (memory tests, guardbands, and error correcting codes (ECC)) in real DRAM chips exhibiting both intermittent and permanent retention failures. Our analysis allows us to quantify the efficacy of recent system-level error mitigation mechanisms that build upon these techniques. We revisit prior works in the context of the experimental data we present, showing that our measured results significantly impact these works' conclusions. We find that mitigation techniques that rely on run-time testing alone [38, 27, 50, 26] are unable to ensure reliable operation even after many months of testing. Techniques that incorporate ECC[4, 52], however, can ensure reliable DRAM operation after only a few hours of testing. For example, VS-ECC[4], which couples testing with variable strength codes to allocate the strongest codes to the most error-prone memory regions, can ensure reliable operation for 10 years after only 19 minutes of testing. We conclude that the viability of these mitigation techniques depend on efficient online profiling of DRAM performed without disrupting system operation.