Cache misses under LRU eviction

An LRU (least-recently-used) cache holds at most 33 items. Starting empty, it receives the access sequence 1, 2, 3, 1, 4, 2, 5, 1.1,\ 2,\ 3,\ 1,\ 4,\ 2,\ 5,\ 1. An access is a miss if the item is not currently cached (a miss on a full cache evicts the least-recently-used item to make room). How many of the 88 accesses are misses?

Show hints (2)+
  1. Track the cache as an ordered list; every hit moves that item to most-recently-used, and a full-cache miss evicts the least-recently-used.
  2. Watch item 11: it hits at step 4 but misses at step 8 because it was evicted at step 7.

Answer

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7

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Asked at: Citadel, SIG

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