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Intermediate Topic 31 of 51 Operating Systems

Page Replacement Algorithms

Policies such as FIFO, LRU and Clock for choosing victims.

How it works

When memory is full and a page fault needs a free frame, the OS chooses a victim page to evict. Common algorithms are FIFO, optimal, which evicts the page used furthest in the future and is only theoretical, LRU, clock, an efficient approximation of LRU with a reference bit, and variants such as second chance and working-set based policies. Dirty pages must be written back before reuse.

Step-by-step walkthrough

  1. All three frames are full.
  2. A new page is needed.
  3. FIFO evicts the page loaded first.
  4. LRU evicts the page unused the longest.
  5. The Clock algorithm approximates LRU with a reference bit.

Where it is used

Operating systems, database buffer managers and caches use these algorithms. Linux uses an approximation of LRU with active and inactive lists.

Common pitfall

FIFO can suffer Belady's anomaly, where more frames cause more faults. True LRU is costly to implement exactly. Scans of large files can evict useful pages, so scan-resistant variants are needed. Poor choices lead to thrashing. Evaluate algorithms with reference strings and fault counts.

Where it fits in the course

Page Replacement Algorithms is topic 31 of 51 in Operating Systems, rated Intermediate. It follows Page Faults. The next topic is Thrashing.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Operating Systems quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Page Replacement Algorithms?
Policies such as FIFO, LRU and Clock for choosing victims. It belongs to Operating Systems and is an intermediate-level topic.
Where do people use Page Replacement Algorithms?
Operating systems, database buffer managers and caches use these algorithms.
What is the most common mistake with Page Replacement Algorithms?
FIFO can suffer Belady's anomaly, where more frames cause more faults.
What should I learn before this topic?
Start with Page Faults, the topic just before it in Operating Systems.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Operating Systems quiz.
Is Page Replacement Algorithms hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn Page Replacement Algorithms?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Page Replacement Algorithms?
Page Faults, Thrashing, Virtual Memory are closely connected. The full list is in the related topics section of this page.
What should I study after Page Replacement Algorithms?
Continue with Thrashing, the next topic in Operating Systems.

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