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Summary of Section 9.1: Swapping

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Standard Swapping

• Swapping allows processes to be temporarily moved from memory to a backing store (secondary storage) and later brought back.
• Purpose:
  • Increases multiprogramming by allowing more processes than available physical memory.
  • Frees memory from idle processes, making space for active ones.
• Challenges:
  • Time-consuming to move entire processes.
  • Not commonly used in modern systems, except in rare cases (e.g., Solaris under extreme memory pressure).

Swapping with Paging

• Modern OS (Linux, Windows) use paging-based swapping instead of moving entire processes.
• Key operations:
  • Page Out → Moves a page to the backing store.
  • Page In → Brings a page back into memory.
• Benefit: More efficient than swapping entire processes.

Swapping in Mobile Systems

• Most mobile OS (iOS, Android) do not support swapping due to:
  • Limited flash memory space.
  • Wear-out issues (flash memory has limited write cycles).
  • Low throughput between RAM and flash.
• Alternatives:
  • iOS: Requests apps to free memory; may terminate apps if necessary.
  • Android: Saves app state before termination to allow quick restarts.

System Performance and Swapping

• Excessive swapping indicates insufficient memory.
• Solutions:
  1. Terminate processes.
  2. Increase physical memory.

Glossary