Every access stream is three streams
Take any real trace and split it into three components:
| Component | What predicts reuse | What it demands |
|---|---|---|
| Stationary popularity: some items are simply hotter, and stay hot | frequency | keep frequent items; use an admission filter |
| Bursty locality: an item is hot for a window and then dead (a session, a job's working set, a dashboard refresh cycle) | recency | keep recent items; promote fast |
| One-shot traffic: scans, backfills, monotonic IDs, each item touched once | nothing, because there is no reuse | refuse to admit |
LRU reads only the second signal (recency). LFU reads only the first (frequency). Neither defends against the third. One sequential scan of a table larger than the cache evicts the whole resident set and replaces it with items that will never be requested again.
LRU's true worst case is a cyclic scan slightly larger than the cache. With 101 items scanned in a loop through an LRU cache of 100, each item is evicted just before it is needed again, so the hit rate is exactly zero. Random eviction on the same loop still delivers about 98%.
This derives the architecture of good caches before we name it: a policy must have at least as many components as the workload has components. You need a recency region, a frequency region, and an admission decision. That is exactly the shape of W-TinyLFU (a small LRU "window" for bursts, a large frequency-governed main region, and a sketch-based admission test between them). The Caffeine cache library later added adaptive resizing of the window, because the mix of the three components drifts during the day. The policy is not clever. It is structurally matched to a three-component stream.
A claim you can test: measure the three fractions for a workload and you can predict which policy family will win without running any of them. (A lab at the end of this mechanism does this.)
In one sentence: three streams (stationary, bursty, one-shot), and a policy needs one component per stream plus a door for the third.