ITPEC FE Morning April 2019 Question 11

A decrease in the cache hit rate — fewer hits send more accesses to the slower main memory.

Average memory access time = hit time + miss rate × miss penalty, or equivalently h × T_c + (1 − h) × T_m with hit rate h, cache access time T_c and main memory access time T_m. Since T_m > T_c, a smaller h shifts weight to T_m and the average rises.

Answer (b)

Why not others:

• •(a) — a faster cache lowers the hit time, so the average falls
• •(c) — a smaller miss penalty makes each miss cheaper, so the average falls
• •(d) — a lower miss rate is a higher hit rate, so the average falls

Key rule: Average access time grows with the hit time, the miss rate and the miss penalty; a lower hit rate is the same thing as a higher miss rate.

Computer Component · AI-written, may contain errors