the compute nodes of the Blue Gene family of supercomputers run CNK (for Compute Node Kernel), a lightweight kernel that runs on each node and supports a single application running for a single user on that node. For the sake of efficient operation, the design of CNK was kept simple and minimal, and it was implemented in about 5,000 lines of C++ code. Physical memory is statically mapped and the CNK neither needs nor provides scheduling or context switching, given that at each point it runs a single application for a single user. By not allowing virtual memory or multi-tasking, the design of CNK aimed to devote as many cycles as possible to application processing. CNK does not even implement file I/O on the compute node, but delegates that to dedicated I/O nodes.
the I/O nodes of the Blue Gene family of supercomputers run INK (for I/O Node Kernel). INK is based on a modified Linux kernel.