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Chapter 10 : Case Study - UNIX - page 27 / 48

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Ceng 334 - Operating Systems

10-27

Two-handed Clock Algorithm (2)

Page daemon has to do two passes with one core map pointer. Pass 1 clears all R bits, second pass removes pages (R bits are set between pass 1 and 2)

Page daemon maintains two pointers into the core map to speed up the process (one pass instead of two) for large memories

When page daemon runs, it first clears the R bit at the front hand, and then checks the R bit at the back hand, after which it advances both hands

Each time the page daemon runs, the hands rotate less than a full revolution, the amount depending on the number of pages needed to reach lotsfree

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