Nonatomic Mutual Exclusion with Local Spinning ∗ ( Extended

We present an N -process local-spin mutual exclusion algorithm, based on nonatomic reads and writes, in which each processperforms Θ(log N) remote memory references to enter and exit its critical section. This algorithm is derived from Yang andAnderson’s atomic tree-based local-spin algorithm in a way that preserves its time complexity. No atomic read/write algorithm with better asymptotic worst-case time complexity (under the remote-memory-references measure) is currently known. Thissuggests that atomic memory is not fundamentally required if one is interested in worst-case time complexity. The same cannot be said if one is interested in fast-path algorithms (in which contention-free time complexity is requiredto be O(1)) or adaptive algorithms (in which time complexity is required to be proportional to the number of contend-ing processes). We show that such algorithms fundamentally require memory accesses to be atomic. In particular, weshow that for any N -process nonatomic algorithm, there exists a single-process execution in which the lone competing pro-cess executes Ω(log N/ log log N) remote operations to enter its critical section. Moreover, these operations must access Ω(√logN/ log logN) distinct variables, which implies that fast and adaptive algorithms are impossible even if caching tech-niques are used to avoid accessing the processors-to-memory interconnection network. ∗Work supported by NSF grants CCR 9972211, CCR 9988327, and ITR 0082866.