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


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


This was a multiple-choice question with the following possible answers:


(A) 4 only 
(B) 1 and 4 only
(C) 1, 2 and 4 only 
(D) 1, 2, 3 and 4


I think L2 cache must be at least as large as the L1 cache but I am confused what the need for writeback is for this cache.
Correct Answer: 1) and 4) only . For inclusion property to hold, every changes in L1 should be made in L2 also. therefore , L1 should be write-through. It is not necessary fr L2 to be write -back. And 4) is mandatory for the same reason i.e L2 would need to hold the contents as same as L1 and as much as ,or may be greater than L1.