GATE CS 2021 Set 2 Q.37 (CS Q.27): Memory Hierarchy: Cache, Main Memory and Secondary Storage

Assume a two-level inclusive cache hierarchy, L1 and L2, where L2 is the larger of the two. Consider the following statements:

$S_1$ : Read misses in a write through L1 cache do not result in writebacks of dirty lines to the L2.

$S_2$ : 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?

$S_1$ is true and $S_2$ is false

$S_1$ is false and $S_2$ is true

$S_1$ is true and $S_2$ is true

$S_1$ is false and $S_2$ is false

Official answer and solution

Official answer: A
From: Official GATE 2021 answer key

Solution

Evaluate the architectural behavior of inclusive cache hierarchies and cache write policies.

Evaluate statement $S_1$ :
In a write-through L1 cache, every write operation immediately updates both the L1 cache block and the corresponding block in the lower level (L2). Consequently, a block in a write-through L1 cache is never dirty (modified relative to L2). When an L1 read miss occurs and a line must be evicted, it is always clean and simply overwritten without needing a writeback to L2. Thus, $S_1$ is true.

Evaluate statement $S_2$ :
Write-allocate (allocating a cache line on a write miss) is traditionally paired with write-back caches to avoid repeated memory writes on subsequent writes to the same block. Write-through caches typically use no-write allocate (write-around) because writing directly to lower memory avoids bringing the block into cache on a miss. Statement $S_2$ claims the opposite, and asserts it as a mandatory requirement ('must'). Thus, $S_2$ is false.

Therefore, $S_1$ is true and $S_2$ is false, which corresponds to option A.

Final answer

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2021 Computer Science and Information Technology - Session 2 (CS2)
Question: 37 of 65, printed as CS Q.27
Section: Computer Science and Information Technology

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