The qthreads API is designed to make using large numbers of threads convenient and easy, and to allow portable access to threading constructs used in massively parallel shared memory environments. The API maps well to both MTA-style threading and PIM-style threading, and is still quite useful in a standard SMP context. The qthreads API provides access to full/empty-bit (FEB) semantics, where every word of memory can be marked either full or empty, and a thread can wait for any word to attain either state.
The qthreads library on an SMP (i.e. the POSIX implementation) is essentially a library for spawning and controlling coroutines: threads with small (4k) stacks. The threads are entirely in user-space and use their blocked/unblocked status as part of their scheduling. The library’s metaphor is that there are many qthreads and several “shepherds”. Shepherds can be thought of as a thread mobility domain; they map to specific processors or memory regions. Qthreads are assigned to specific shepherds and do not migrate unless directed to migrate. The API includes utility functions for making threaded loops, sorting, and similar operations convenient.
The Qthread library was developed to explore innovations in highly concurrent systems where the ultimate system either does not exist, or is sufficiently hard to obtain that development of software for the system becomes difficult.
Development is currently hosted on GitHub: https://github.com/Qthreads/qthreads.