Concurrent Reading While Writing

Revised February, 1981The problem of asynchronous processes reading shared data while the data is being modified by another process is considered. This problem differs from the standard readers/writers problem in that concurrent reading while writing is allowed. The model used here strongly limits the use and size of the shared variables. If multiple copies of the shared data are allowed, then simple, efficient solutions are found. In general, solutions which are more time efficient by avoiding waiting are seen to require more copies of the shared data. The number of copies used by all algorithms is shown to be the best possible. The main solution demonstrates that any system of processes which uses large distributed variables can be strongly simulated by a system which uses only binary distributed variables