Variable Weight Processes with Flexible Shared Resources

Traditional UNIX processes are inadequate for representing multiple threads of control in parallel programs. They are inflexible in their resource allocation, unable to cleanly share system resources, and they carry a heavy overhead. Some new operating systems, such as MACH, split the process into multiple light-weight threads of execution and a task which defines their resource set. This paper addresses the same problem within the confines of UNIX process semantics. It partitions the existing concept of a process into a new “variable weight ” process and several independent “system resource descriptors”. Processes pay creation and maintenance costs only for resources they wish to keep private. Surprisingly few changes to the kernel are needed to achieve this effect. The design can properly be considered as a simple concurrency extension to UNIX. Variable-weight processes also enhance the effectiveness of the cooperating process paradigm and so are broadly applicable to both uniprocessor and multiprocessor UNIX implementations. A prototype kernel has been implemented on Multimax TM, a shared-memory multiprocessor. Preliminary experience indicates that relatively few changes to the process structures in UNIX make this strategy incrementally applicable to a range of UNIX variants