Beyond threads: resource sharing in UNIX

UNIX provides a programming model for the user which gives an illusion of multi-processing. On uniprocessors, this illusion works well, providing communication paths, firewalls between processes and a simple programming environment. Unfortunately, the normal UNIX model does not provide the capabilities to take full advantage of modern multiprocessor hardware. This results from a design which uses data queueing and pre-emption to provide the multiprocessing illusion, which are unnecessary on a true multi-processor. The recent proposed addition of threads to CMU’s MACH kernel shows that there are other effective programming models that may be supported in UNIX as well. This model provides for sharing the virtual address space of a process by breaking the process into several lightweight contexts that may be manipulated more quickly than a normal process. Unfortunately, this model raises questions about support for normal UNIX semantics, as well as thread scheduling performance and kernel overhead. This paper describes a programming model which goes beyond the simple concept o