Swapping in Memory Management

Definition

Swapping is the technique of temporarily moving a process (whole or part) from main memory (RAM) to secondary storage (swap space/backing store) and bringing it back later for execution.Introduced in early time-sharing systems (Atlas, UNIX) to support multiprogramming beyond physical memory size.

Stored image on disk is called swap-out; retrieving is swap-in (roll-out / roll-in).

How it Works

1. Scheduler detects low free frames or blocked process waiting for I/O.
2. Swapper (mid-term scheduler) selects victim process → copies its entire address space (code, data, stack, PCB) to contiguous swap area on disk.
3. Page tables / base-limit registers updated, frames freed.
4. Later when memory available and process scheduled, swapper reads image back (possibly at different physical address → requires relocation / dynamic binding via relocation register).

Modern systems swap pages/segments, not whole processes (demand paging is a lazy swap).

Diagram: Main Memory [P1|P2|P3|hole] ↔ Disk Swap Area [P4(swapped)] ↔ Scheduler

Why Used? (Objectives)

• Multiprogramming degree: Run more processes than fits in RAM. Example $8$ processes $\times 200KB =1.6MB$ on $1MB$ RAM via time-multiplexing.
• Memory compaction / fragmentation removal: Swapping out creates large contiguous free blocks, reducing external fragmentation.
• Priority & scheduling: Swap out low-priority/blocked processes to give room to urgent ones (mid-term suspend).
• Fairness & time-sharing: Quantum expired → swap out to let next process in.
• Emergency handling: Low-memory handler frees RAM quickly.
• I/O overlap & backing store for paging: Pages not resident stay on disk.

Types

• Whole-process swapping: Simple but heavy I/O (entire address space).
• Page swapping (paging): Swap only pages, more efficient, foundation for virtual memory.
• Segment swapping: Similar for segmented systems.

Cost & Issues

• Transfer time: Main cost $T_{swap}=2\times (size/transfer-rate) + seek/latency$. e.g. $10MB/100MBps=100ms$. So swap only if process idle for longer.
• Blocked I/O: Process with pending I/O or pinned pages cannot be swapped.
• Thrashing risk: Excessive swapping degrades CPU utilization → need working set / locality.

Today pure swapping rare; virtual memory + demand paging is generalized swapping with finer granularity, and swap file/partition remains as overflow store.