Approximating LRU

Learn how a system approximates LRU with the help of use bits and the clock algorithm.

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Use bits

As it turns out, the answer is yes: approximating LRU is more feasible from a computational-overhead standpoint, and indeed it is what many modern systems do. The idea requires some hardware support, in the form of a use bit (sometimes called the reference bit), the first of which was implemented in the first system with paging, the Atlas one-level store“One-level Storage System” by T. Kilburn, D.B.G. Edwards, M.J. Lanigan, F.H. Sum- ner. IRE Trans. EC-11:2, 1962. Although Atlas had a use bit, it only had a very small number of pages, and thus the scanning of the use bits in large memories was not a problem the authors solved.. There is one use bit per page of the system, and the use bits live in memory somewhere (they could be in the per-process page tables, for example, or just in an array somewhere). ...