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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