27. Consider a system that has multiple processors where each processor has its own cache, but main memory is shared among all processors.

a) Which cache write policy would you use?

b) The Cache Coherency Problem. With regard to the system just described, what problems are caused if a processor has a copy of memory block A in its cache and a second processor, also having a copy of A in its cache, then updates main memory block A? Can you think of a way

(perhaps more than one) of preventing this situation, or lessening its effects?