For inclusion to hold between two cache levels L1 and L2 in a multi-level cache hierarchy, which of the following are necessary?

I. L1 must be a write-through cache
II. L2 must be a write-through cache
III. The associativity of L2 must be greater than that of L1
IV. The L2 cache must be at least as large as the L1 cache 

(A) IV only
(B) I and IV only
(C) I, III and IV only
(D) I, II, III and IV


Answer: (B)

Explanation: Answer is (B), i.e., (i) and (iv) are true.

Because inclusion says the L2 cache should be Superset of L1 cache. If \”Write Through update\” is not used (and \”Write Back update\” is used ) at L1 cache, then the modified data in L1 cache will not be present in L2 cache for some time unless the block in L1 cache is replaced by some other block. Hence \”Write Through Update \” should be used.

Associativity doesn\’t matter.

L2 cache must be at least as large as L1 cache, since all the words in L1 are also is L2.


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