Editing Release consistency (section)

==  Lazy release consistency ==
[[File:Eager RC.png|thumb|400x400px|Write propagation in release consistency model]]
Lazy release consistency is a further optimization of release consistency. It assumes that the thread executing an acquire access does not need the values written by other threads until the acquire access has completed. Hence, all behavior of coherence can be delayed and timing for write propagation can be tweaked.<ref>[http://dl.acm.org/citation.cfm?id=139676 ''Lazy release consistency for software distributed shared memory''] by Pete Keleher, Alan L. Cox, and Willy Zwaenepoel published in Proceeding ISCA '92 Proceedings of the 19th annual international symposium on Computer architecture
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=== Example ===
Consider the scenarios described in the image to the right. This case shows when write propagation is performed on a cache-coherent system based on the release consistency model. The variable ''datum'' is completely propagated before the propagation of ''datumIsReady.'' But the value of ''datum'' is not needed until after the acquire synchronization access in P1 and it can be propagated along with ''datumIsReady'' without harming the result of the program.

[[File:Lazy RC.png|thumb|400x400px|Write propagation in lazy release consistency model]]
The second image displays what is the case when lazy release consistency is applied. Considering this scenario, all values written ahead of the release synchronization are delayed and propagated together with the propagation of the release access itself. Hence, ''datum'' and ''datumIsReady'' are propagated together at the release point.

"TreadMarks"<ref>[http://dl.acm.org/citation.cfm?id=1267084 ''TreadMarks: distributed shared memory on standard workstations and operating systems''] by Pete Keleher, Alan L. Cox, [[Sandhya Dwarkadas]] and Willy Zwaenepoel published in WTEC'94 Proceedings of the USENIX Winter 1994 Technical Conference on USENIX Winter 1994 Technical Conference</ref> is an actual application of lazy release consistency.

=== Performance improvement over release consistency ===
Lazy release consistency can outperform release consistency in certain cases. If there is a system with little bandwidth between processors or it suffers badly from the higher overheads because of frequent propagation of small block of data versus infrequent large data block propagation, LRC can really help the performance.

Consider a system employs a software level shared memory abstraction rather than an actual hardware implementation. In this system, write propagation is executed at a page granularity, which makes it extremely expensive to propagate a whole page when only one block in this page is modified. Therefore, write propagation is delayed until a release synchronization point is reached and the entire page will be modified at this time and the whole page will be propagated.

=== Drawback ===
LRC requires performing write propagation in bulk at the release point of synchronization. Propagating such a large number of writes altogether will slow down the release access and the subsequent acquire access. Hence it can hardly improve the performance of a hardware cache coherence system.