{
  "slug": "belady-anomaly",
  "term": "Belady anomaly",
  "aliases": [
    "Belady's anomaly",
    "FIFO capacity anomaly"
  ],
  "definition": "A Belady anomaly occurs when a replacement policy produces more misses with a larger cache than with a smaller cache on the same reference string and initial-state convention.",
  "note": "Belady, Nelson and Shedler studied the FIFO case in 1969. [belady]\n\nThe 2010 unboundedness result varies capacities and reference strings; it is not an unbounded ratio for fixed capacities. [fifo-unbounded]",
  "related": [
    "fifo-replacement",
    "cache-inclusion-property",
    "lru-replacement"
  ],
  "citations": [
    {
      "id": "belady",
      "title": "Belady, Nelson and Shedler. An anomaly in space-time characteristics of certain programs running in a paging machine. CACM, 1969.",
      "url": "https://research.ibm.com/publications/an-anomaly-in-space-time-characteristics-of-certain-programs-running-in-a-paging-machine",
      "accessed": "2026-10-17"
    },
    {
      "id": "fifo-unbounded",
      "title": "Peter Fornai and Antal Ivanyi. FIFO anomaly is unbounded. Acta Univ. Sapientiae, Informatica 2(1), 2010, pp. 80-89; sections 2-5, arXiv:1003.1336v2.",
      "url": "https://arxiv.org/pdf/1003.1336v2",
      "accessed": "2026-10-17"
    },
    {
      "id": "hesela",
      "title": "Hesela: finite cache-capacity replay, with full traces and assumptions.",
      "url": "https://hesela.com/analyses/sieve-cache-eviction-traces/#capacity",
      "accessed": "2026-10-17"
    }
  ],
  "status": "research",
  "advantages": [],
  "limitations": [],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-10-17",
  "provenance": [
    "Hesela replays finite examples, not real hardware or a workload survey. [hesela]",
    "Original ACM full text was unavailable; historical metadata was checked against the publication record. [belady]"
  ]
}