Customized memory modules in multi-tenant provider systems

A computing system providing virtual computing services may maintain a fleet of servers that host virtual machine instances having a wide variety of types and configurations. A service provider may rent processor and memory capacity by defining and offering various virtual machine instances to clients. Each virtual machine instance may include one or more virtual CPUs and a fixed amount of virtualized memory allocated to each virtual CPU, dependent on a predefined ratio between virtual CPU capacity and virtualized memory capacity for the instance type. Each server may include a custom, non-standard sized physical memory module containing memory devices of multiple technologies, types, or sizes on the same printed circuit board. By including custom memory modules, rather than relying only on standard memory modules, the service provider system may implement virtual machines having finer grained options for processor and memory capacity combinations, and may avoid stranding rentable resources.