On the Tamability of the Location Consistency Memory Model

Charles Wallace, Guy Tremblay, and José Nelson Amaral

Gao and Sarkar have proposed Location Consistency (LC), the weakest memory model described in the literature to date for shared memory architectures. The advantage of LC is that it does not require invalidation/update messages across processors, thus dispensing with cache snooping or directories. Gao and Sarkar have argued that, although LC is strictly weaker than Release Consistency (RC), it is equivalent to RC for all programs that are data-race free. However the LC specification is silent about which reorderings of consistency related operations and program statements are allowed. In this paper we argue that under permissive reordering rules, LC is not equivalent to RC, even for data race free programs. On the other hand, if strict reordering rules are followed, then LC is equivalent to RC for data race free programs but the distinction between LC and RC is greatly diminished and many of the advantages of LC are lost.

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