Summary: Nonatomic Mutual Exclusion with Local Spinning #
(Extended Abstract)
James H. Anderson and Yong­Jik Kim
Department of Computer Science
University of North Carolina at Chapel Hill
{anderson, kimy}@cs.unc.edu
ABSTRACT
We present an N­process local­spin mutual exclusion algorithm, based on nonatomic reads and writes, in which each process
performs #(log N) remote memory references to enter and exit its critical section. This algorithm is derived from Yang and
Anderson's atomic tree­based local­spin algorithm in a way that preserves its time complexity. No atomic read/write algorithm
with better asymptotic worst­case time complexity (under the remote­memory­references measure) is currently known. This
suggests that atomic memory is not fundamentally required if one is interested in worst­case time complexity.
The same cannot be said if one is interested in fast­path algorithms (in which contention­free time complexity is required
to 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, we
show that for any N­process nonatomic algorithm, there exists a single­process execution in which the lone competing pro­
cess executes #(log N/ log log N) remote operations to enter its critical section. Moreover, these operations must access
#( # log N/ log log N) distinct variables, which implies that fast and adaptive algorithms are impossible even if caching tech­
niques are used to avoid accessing the processors­to­memory interconnection network.
# Work supported by NSF grants CCR 9972211, CCR 9988327, and ITR 0082866.

Source: Anderson, James - Department of Computer Science, University of North Carolina at Chapel Hill

Collections: Computer Technologies and Information Sciences