Design Issues
Granularity and page replacement are the two important issues to be considered in the design of a DSM system. They are important because the efficiency of DSM depends on the effectiveness of the size chosen for granularity and the protocol used for page replacement.
Granularity
Granularity refers to the size of the shared memory unit. A page size of that is a multiple of the size provided by the underlying hardware or the memory management system allows for the integration of DSM and the memory management
systems. By integrating DSM with the underlying memory management system, a DSM system can take advantage of the built in protection mechanism to detect incoherent memory references, and use built in fault handlers to prevent and recover from inappropriate references.
A large page size for the shared memory unit will take advantage of the locality of reference exhibited by processes. By transferring large pages, less overhead is incurred due to page size, but there is greater chance for contention to access a page by many processes. Smaller page sizes are less apt to cause contention as they reduce the
likelihood of false sharing. False sharing of a page occurs when two different data items, not shared but accessed by two different processes, are allocated to a single page. So the protocols that adapt to a granularity size that is appropriate to the sharing pattern will perform better than those protocols that make use of a static granularity size.
Page Replacement
A memory management system has to address the issue of page replacement because the size of physical memory is limited. In DSM systems that support data movement, traditional methods such as least recently used (LRU) cannot be used directly. Data may be accessed in different modes such as shared, private, read-only, writable, etc., in DSM systems. To avoid a degradation in the system performance, a page replacement policy would have to take the page access modes into consideration. For instance, private
pages may be replaced before shared pages, as shared pages would have to be moved over the network, possibly to their owner. Read-only pages can simply be deleted as their owners will have a copy. Thus the LRU policy with classes is one possible strategy to handle page replacement. Once a page is selected for replacement, the DSM system must ensure that the page is not lost forever. One option is to swap the page onto disk memory. However, if the page is a replica and is not owned by the node, it can be sent to the owner node.
DSM Home Subway Coherence Various
Page Map Protocols Systems