A cache memory that has a hit rate of 0.8 has an access latency 10 ns and miss…
2022
A cache memory that has a hit rate of 0.8 has an access latency 10 ns and miss penalty 100 ns. An optimization is done on the cache to reduce the miss rate. However, the optimization results in an increase of cache access latency to 15 ns, whereas the miss penalty is not affected. The minimum hit rate (rounded off to two decimal places) needed after the optimization such that it should not increase the average memory access time is _____________.
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Correct answer: 0.85
Key idea: average memory access time (AMAT) = cache access latency + miss rate × miss penalty.
Initial AMAT = 10 + (1 - 0.8) × 100 = 10 + 20 = 30 ns.
After optimization AMAT = 15 + (1 - h) × 100. We require 15 + (1 - h) × 100 ≤ 30.
Solve: (1 - h) × 100 ≤ 15 ⇒ 1 - h ≤ 0.15 ⇒ h ≥ 0.85.
Minimum hit rate needed = 0.85 (rounded to two decimal places).
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