Lineup is a user-space, cooperative thread scheduler that runs green-threads (user-level threads). It supports many synchronization primitives (mutex, rwlock, conditional variables, barriers etc.), thread-local storage, and has some basic support for multi-threading. It uses fringe for compiler-assisted context-switching. The scheduler code is found in lib/lineup.


The kernel can notify the scheduler about events through an up-call mechanism, for example to notify about more available cores (or removal of cores), to forward device interrupts, or page-faults from kernel to user-space.

The mechanism for this is inspired by scheduler activations: The kernel and user-space program agree on a common save area to store a CPU context (on a per-core basis). If an event arrives at the kernel, it will save the current CPU context (registers etc.) in the agreed upon save-area and resume the process with a new (mostly empty) context that invokes the pre-registered upcall handler instead. The upcall handler gets all information about the event that triggered the interruption through function arguments so it can then take appropriate measures to react. After the event is handled, the upcall handler can read the previous context (from before the interruption) from the common save area and decide to resume where computation left off before the upcall (or decide not to continue with this context).