GATE CS 2016 Set 2 — Question 50
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Computer Organization & Architecture → Memory Hierarchy & Cache → Cache Numericals
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Question
A file system uses an in-memory cache to cache disk blocks. The miss rate of the cache is shown in the figure. The latency to read a block from the cache is 1 ms and to read a block from the disk is 10 ms. Assume that the cost of checking whether a block exists in the cache is negligible. Available cache sizes are in multiples of 10 MB.
The smallest cache size required to ensure an average read latency of less than 6 ms is ______ MB.

Correct answer
64 to 64
Solution
Let be the hit rate and be the miss rate ().
The average read latency is given by:Given:
ms
ms
We require ms.Substituting the values:So, the miss rate must be less than .Looking at the graph:
The average read latency is given by:Given:
ms
ms
We require ms.Substituting the values:So, the miss rate must be less than .Looking at the graph:
- At 10 MB, Miss rate
- At 20 MB, Miss rate
- At 30 MB, Miss rate
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