Timing-based mutual exclusion

The benefits that can be obtained by using timing information in mutual exclusion algorithms are examined. A simple and efficient timing-based mutual exclusion algorithm is given. This algorithm always guarantees mutual exclusion (i.e. even when run asynchronously) and also avoids deadlock in case certain (realistic) inexact timing constraints are met. The algorithm uses only two shared read/write registers (a total of log n+1 bits), thus overcoming the n register lower bound for asynchronous algorithms. It is proved that the problem cannot be solved with only one shared register, so that this algorithm is optimal in terms of the number of registers. A lower bound is proved for the time complexity of any deadlock-free mutual exclusion protocol, as a function of the number of shared registers it employs. This bound shows that the algorithm described is near optimal, in terms of time complexity. It is shown that two natural ways of weakening the timing assumptions lead (unfortunately) to an n register lower bound.<>