Memory Management in Operating Systems: Memory Fragmentation Issues and Solutions

Memory Management in Operating Systems: Memory Fragmentation Issues and Solutions

Description
Memory fragmentation refers to the phenomenon where memory space is divided into many small, non-contiguous blocks, making it unavailable for allocation to processes. There are two types of memory fragmentation: external fragmentation and internal fragmentation. External fragmentation refers to free space between allocated units, while internal fragmentation refers to unused space within an allocated unit. Fragmentation reduces memory utilization and can even lead to memory allocation failures.

I. Types and Causes of Memory Fragmentation

  1. External Fragmentation

    • Definition: Scattered free areas in memory that are collectively sufficient but cannot be merged for use.
    • Cause: Frequent memory allocation and deallocation cause free memory to be split into small blocks.
    • Example: If there are three free blocks in memory (10KB, 5KB, 8KB), but a process requires 15KB of contiguous space, allocation fails.

  2. Internal Fragmentation

    • Definition: The portion within a memory block allocated to a process that is not actually used.
    • Cause: Memory allocation mechanisms allocate in fixed units (e.g., 4KB pages), but the actual requirement of a process may be smaller than this unit.
    • Example: A process only needs 2KB, but the system allocates a 4KB page, leaving 2KB as internal fragmentation.

II. Methods to Solve External Fragmentation

  1. Compaction

    • Principle: Move allocated memory blocks to merge free memory into a contiguous region.
    • Steps:
      1. Suspend all process execution.
      2. Move allocated memory blocks towards one end.
      3. Update the physical address mapping of processes (requires hardware support for relocation registers).
    • Disadvantages: High overhead for moving memory and requires pausing the entire system (Stop-the-World).

  2. Partition Allocation Strategy Optimization

    • First Fit: Searches from the starting address of memory for the first sufficiently large free block.
      • Advantage: Fast allocation.
      • Disadvantage: May produce many small fragments.
    • Best Fit: Selects the smallest free block that is closest in size to the requirement.
      • Advantage: Reduces the splitting of large memory blocks.
      • Disadvantage: May leave tiny, hard-to-use fragments.
    • Worst Fit: Selects the largest free block for allocation.
      • Purpose: Avoids producing excessively small fragments.
      • Disadvantage: May compromise the availability of large memory blocks.

  3. Non-Contiguous Memory Allocation

    • Paging:
      • Divides memory into fixed-size pages (e.g., 4KB), allowing process memory to be stored dispersedly.
      • Maps virtual addresses to physical pages via a page table, avoiding external fragmentation (though internal fragmentation remains).
    • Segmentation:
      • Allocates memory by logical units (e.g., code segment, data segment) with variable segment lengths.
      • Requires hardware segment table support and may produce external fragmentation (requiring compaction or combined segmentation and paging).

III. Methods to Solve Internal Fragmentation

  1. Reduce Allocation Unit Size

    • For example, change page size from 4KB to 512B to reduce unused space.
    • Disadvantage: Increases page table size and address translation overhead.

  2. Dynamic Allocation Optimization

    • Slab Allocator (for kernel objects):
      • Reserves memory pools for commonly used objects (e.g., task structures) and allocates precisely by object size.
      • Avoids internal fragmentation caused by general allocation strategies.
    • Buddy System:
      • Divides memory into blocks sized as powers of two and allocates the closest block size as needed.
      • Internal fragmentation is controllable but may still exist (e.g., requesting 3KB results in allocating 4KB).

IV. Comparison of Comprehensive Solutions

  • Paging Mechanism: Eliminates external fragmentation entirely, but internal fragmentation is unavoidable.
  • Segmentation Mechanism: Reduces internal fragmentation (allocates as needed) but requires handling external fragmentation.
  • Combined Segmentation and Paging: Manages logical units via segmentation and further divides each segment into pages, balancing fragmentation and flexibility.

Summary
Memory fragmentation is a core challenge in memory management. Strategies must be chosen based on the scenario: real-time systems may use compaction techniques, while general-purpose systems commonly use paging with local optimizations (e.g., Slab allocator). Understanding the causes of fragmentation and the solutions helps in designing efficient memory allocation algorithms.