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kayvonf

Question: What is write serialization? Someone take a crack at explaining why the MSI protocol ensures write serialization.

Incanam

Write serialization means that all processors see the same ordering of reads and writes, ensuring that there can be an overall timeline of operations.

We said in lecture that the interconnect ensures reads and writes are ordered by the order that they appear on the interconnect. The trickier case is the writes on a cache line held exclusively by one processor. The key is that only this processor, P, can write to it while it has exclusive access, so the only ordering that exists is the one that P sees. When another processor requests to read the cache line, P, flushes the line meaning all other processors now see the same order of writes to that line that P did. Only then can the requesting processor read the line. Since the flushing and bus communication is done over the interconnect (which ensures an ordering), we are ensured that all those writes to the cache line by P will fit in some overall ordering that will be consistent for all processors.