It usually takes longer for a processor to access shared

memory than to access its own private memory because of

contention for the processor-to-memory connections and because

of other overheads associated with ensuring synchronised

access. Computers using shared memory usually have some kind

of local cache on each processor to reduce the number of

accesses to shared memory. This requires a cacheconsistency protocol to ensure that one processor's cached

copy of a shared memory location is invalidated when another

processor writes to that location.