Reliability-Aware Runahead

Decreasing voltage levels and continued transistor scaling have drastically increased the chance of a processor bit encountering a soft error. We find that the microarchitecture state in an out-of-order core is vulnerable to soft errors especially while waiting for data to return from memory. The severity of the problem is further aggravated by the increasingly large size of microarchitecture state with every new processor generation. Prior solutions are ineffective as they incur too high overhead in terms of chip area, energy consumption and/or performance.In this paper, we make the observation that runahead execution, which was originally conceived to improve performance, also improves soft-error reliability as an unintended side effect. While the state-of-the-art runahead technique, Precise Runahead Execution (PRE), leads to substantial performance improvements, reliability is suboptimal still. We propose Reliability-Aware Runahead (RAR) which substantially improves soft-error reliability over the current state-of-the-art by rendering the microarchitecture state non-vulnerable during runahead execution and by initiating runahead execution early. Across a set of memory-intensive applications — the primary target for runahead execution — RAR improves the mean-time-to-failure (MTTF) by on average 4.8× (and up to 35.8×) relative to an out-of-order baseline while at the same time improving performance by 33.5% on average (and up to 2.6×). Across a broader set of compute- and memory-intensive benchmarks, RAR improves MTTF by on average 2.5× while at the same time improving performance by 11.9% on average. We explore the runahead design space and conclude that RAR is the only design point that improves both reliability and performance by such a significant margin. We find that RAR is more effective for increasingly large processor architectures, making RAR an effective microarchitecture technique for future high-reliability high-performance microprocessors.