  1.5 billion smartphones are sold annually, and most are decommissioned less
than two years later. Most of these unwanted smartphones are neither discarded
nor recycled but languish in junk drawers and storage units. This computational
stockpile represents a substantial wasted potential: modern smartphones have
increasingly high-performance and energy-efficient processors, extensive
networking capabilities, and a reliable built-in power supply. This project
studies the ability to reuse smartphones as "junkyard computers." Junkyard
computers grow global computing capacity by extending device lifetimes, which
supplants the manufacture of new devices. We show that the capabilities of even
decade-old smartphones are within those demanded by modern cloud microservices
and discuss how to combine phones to perform increasingly complex tasks. We
describe how current operation-focused metrics do not capture the actual carbon
costs of compute. We propose Computational Carbon Intensity -- a performance
metric that balances the continued service of older devices with the
superlinear runtime improvements of newer machines. We use this metric to
redefine device service lifetime in terms of carbon efficiency. We develop a
cloudlet of reused Pixel 3A phones. We analyze the carbon benefits of deploying
large, end-to-end microservice-based applications on these smartphones.
Finally, we describe system architectures and associated challenges to scale to
cloudlets with hundreds and thousands of smartphones.
