Книга: Distributed operating systems

6.4.2. Replication

6.4.2. Replication

One improvement to the basic system that can improve performance considerably is to replicate chunks that are read only, for example, program text, readonly constants, or other read-only data structures. For example, if chunk 10 in Fig. 6-25 is a section of program text, its use by processor 1 can result in a copy being sent to processor 1, without the original in processor 2's memory being disturbed, as shown in Fig. 6-25(c). In this way, processors 1 and 2 can both reference chunk 10 as often as needed without causing traps to fetch missing memory. 


Fig. 6-25. (a) Chunks of address space distributed among four machines. (b) Situation after CPU 1 references chunk 10. (c) Situation if chunk 10 is read only and replication is used.

Another possibility is to replicate not only read-only chunks, but all chunks. As long as reads are being done, there is effectively no difference between replicating a read-only chunk and replicating a read-write chunk. However, if a replicated chunk is suddenly modified, special action has to be taken to prevent having multiple, inconsistent copies in existence. How inconsistency is prevented will be discussed in the following sections. 

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