Cache Eviction — LRU vs LFU vs FIFO

A tiny fixed-size cache and a stream of requests. Watch LRU, LFU and FIFO each evict a different victim when the cache fills — and compare their hit ratios on the same workload to see why the policy matters.

A 4-slot cache. Request keys (or run the demo) and watch each policy choose a different victim when the cache is full. Switch the policy and replay the same demo to compare hit ratios.

Cache (0/4)

empty

request:

Hits

Misses

Hit ratio

Run the demo under each policy and compare the hit ratio — same 10 requests, different evictions, different outcomes.

What just happened

▹A cache has limited memory, so when it fills, an eviction policy decides which entry to drop. The same workload gets different hit ratios depending on that choice.
▹LRU evicts what hasn't been touched in the longest time (great for recency-skewed traffic). LFU evicts the least-popular key (great when some keys are perennially hot). FIFO just drops the oldest insert — simple, but ignores how the data is actually used.
▹Watch the metadata on each slot: LRU tracks last-used time, LFU tracks a frequency count, FIFO tracks insertion order. That's literally the only difference — what number they sort the victim by.