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n Vahdat (Google) Session: Fun Hardware — 11am, 2021 June 1 We are accustomed to thinking of computers as fail-stop, especially the cores that execute instructions, and most system software implicitly relies on that assumption. During most of the VLSI era, processors that passed manufacturing tests and were operated within specifications have insulated us from this fiction. As fabrication pushes towards smaller feature sizes and more elaborate computational structures, and as increasingly specialized instruction-silicon pairings are introduced to improve performance, we have observed ephemeral computational errors that were not detected during manufacturing tests. These defects cannot always be mitigated by techniques such as microcode updates, and may be correlated to specific components within the processor, allowing small code changes to effect large shifts in reliability. Worse, these failures are often “silent” – the only symptom is an erroneous computation. We refer to a core that develops such behavior as “mercurial.” Mercurial cores are extremely rare, but in a large fleet of servers we can observe the disruption they cause, often enough to see them as a distinct problem – one that will require collaboration between hardware designers, processor vendors, and systems software architects. This paper is a call-to-action for a new focus in systems research; we speculate about several software-based approaches to mercurial cores, ranging from better detection and isolating mechanisms, to methods for tolerating the silent data corruption they cause.