Assume a two-level inclusive cache hierarchy, L1 and L2, where L2 is the…
2021
Assume a two-level inclusive cache hierarchy, L1 and L2, where L2 is the larger of the two. Consider the following statements.
S1: Read misses in a write through L1 cache do not result in writebacks of dirty lines to the L2
S2: Write allocate policy must be used in conjunction with write through caches and no-write allocate policy is used with writeback caches.
Which of the following statements is correct?
- A.
S1 is true and S2 is false
- B.
S1 is false and S2 is true
- C.
S1 is true and S2 is true
- D.
S1 is false and S2 is false
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Correct answer: A
Answer: S1 is true and S2 is false.
Why S1 is true: In a write-through L1 cache every write is forwarded to L2 (and/or memory) immediately, so L1 does not retain dirty lines. Therefore, a read miss in L1 cannot cause a writeback of a dirty line from L1 to L2.
Why S2 is false: The statement asserts a mandatory pairing of allocation policy and write policy, but that is not correct. Typical conventions are: write-back is often paired with write-allocate (so writes allocate the block on a miss) and write-through is often paired with no-write-allocate (to avoid bringing blocks in just for a write), but these are design choices rather than strict rules.
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