To mitigate climate change, there has been a recent focus on reducing computing's carbon emissions by shifting its time and location to when and where lower-carbon energy is available. However, despite the prominence of carbon-aware spatiotemporal workload shifting, prior work has only quantified its benefits in narrow settings, i.e., for specific workloads in select regions. As a result, the potential benefits of spatiotemporal workload scheduling, which are a function of both workload and energy characteristics, are unclear. To address the problem, this paper quantifies the upper bound on the benefits of carbon-aware spatiotemporal workload shifting for a wide range of workloads with different characteristics, e.g., job duration, deadlines, SLOs, memory footprint, etc., based on hourly variations in energy's carbon-intensity across 123 distinct regions, including cloud regions, over a year. Notably, while we find that some workloads can benefit from carbon-aware spatiotemporal workload shifting in some regions, the approach yields limited benefits for many workloads and cloud regions. In addition, we also show that simple scheduling policies often yield most of the benefits. Thus, contrary to conventional wisdom, i) carbon-aware spatiotemporal workload shifting is likely not a panacea for significantly reducing cloud platforms' carbon emissions, and ii) pursuing further research on sophisticated policies is likely to yield little marginal benefits.

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