This paper examines the amount of communication that is required for performing mutual exclusion. It is assumed that n processors communicate via accesses to a shared memory that is physically distributed among the processors. We consider the possibility of creating a scalable mutual exclusion protocol that requires only a constant amount of communication per access to a critical section. We present two main results, First, we show that there does not exist a scalable mutual exclusion protocol that uses only read and write operations. This result solves an open problem posed by Yang and Anderson, Second, we prove that the same result holds even if test-and-set, compare-andswap, load-and-reserve and store-conditional operations are allowed in addition to read and write operations. Our results hold even if an amortized analysis of communication costs is used, an arbitrary amount of memory is available, and the processors have coherent caches.

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