Memory Allocation Algorithms in Operating Systems (Continuous Memory Allocation)

Memory Allocation Algorithms in Operating Systems (Continuous Memory Allocation)

Description
Continuous memory allocation is a fundamental method of operating system memory management, requiring that the memory needed for a process to run must be located in a contiguous memory region. When multiple processes need to be loaded into memory, the operating system must select suitable free memory blocks to allocate to processes. This involves three common allocation algorithms: First Fit, Best Fit, and Worst Fit. Understanding the differences, strengths, and weaknesses of these algorithms helps in grasping the causes of memory fragmentation and the strategies to address it.

Solution Process

  1. Problem Background

    • Memory is divided into a system area and a user area; the user area holds processes.
    • Free memory blocks are recorded via a "free partition table" or "free partition list" (containing start address, size, and status).
    • When a process requests memory, the operating system must allocate a sufficiently large block from the free blocks. This allocation may produce fragmentation (external fragmentation).

  2. First Fit Algorithm

    • Core Idea: Sequentially search from the low memory address and select the first free block that can satisfy the process size.
    • Implementation Method:
      1. Maintain a free partition list (sorted by increasing address).
      2. Traverse the list from the beginning to find the first free block with size ≥ requested size.
      3. If the block is significantly larger than needed, split it into two parts: one allocated to the process, and the remainder retained as a new free block.
    • Example:
      Assume free block list: [Address 0, Size 100KB] → [Address 200KB, Size 50KB] → [Address 300KB, Size 80KB].
      Process requests 30KB:
      • Check the first block (100KB ≥ 30KB), allocate 30KB, leaving 70KB as a new free block (starting at address 30KB).
    • Characteristics:
      • Advantages: Fast allocation, preserves large free blocks at high addresses.
      • Disadvantages: Low addresses prone to fragmentation (dense small free blocks).

  3. Best Fit Algorithm

    • Core Idea: Select the smallest free block that can satisfy the process requirement to reduce waste from splitting large blocks.
    • Implementation Method:
      1. Maintain a free partition list (sorted by increasing size).
      2. Traverse the list to find the first free block with size ≥ requested size (i.e., the smallest feasible block).
      3. After allocation, if the remaining space is too small (e.g., below a threshold), the entire block may be allocated to avoid micro-fragmentation.
    • Example:
      Free blocks sorted by size: [50KB] → [80KB] → [100KB].
      Process requests 30KB:
      • Select the 50KB block (smallest feasible), leaving 20KB after allocation.
    • Characteristics:
      • Advantages: Tends to preserve large memory blocks.
      • Disadvantages: Produces many tiny fragments, higher traversal overhead.

  4. Worst Fit Algorithm

    • Core Idea: Select the largest free block for allocation, so that the remaining block after splitting is still sufficiently large.
    • Implementation Method:
      1. Maintain a free partition list (sorted by decreasing size).
      2. Directly check the first block (largest block); if it meets the requirement, allocate it.
    • Example:
      Free blocks sorted by size: [100KB] → [80KB] → [50KB].
      Process requests 30KB:
      • Select the 100KB block, leaving 70KB after allocation (still a relatively large block).
    • Characteristics:
      • Advantages: Reduces production of tiny fragments.
      • Disadvantages: Large memory blocks may be quickly split,不利于大型进程的申请 (not conducive to requests from large processes).

  5. Algorithm Comparison and Fragmentation Issues

    • External Fragmentation: Cannot be completely avoided by any algorithm; requires resolution via "compaction" (moving processes to merge fragments).
    • Efficiency: First Fit is most commonly used in practice (balances speed and fragmentation); Best Fit tends to produce unusable small fragments; Worst Fit suits medium-load scenarios.
    • Optimization: Modern operating systems often combine non-contiguous allocation (e.g., paging) to circumvent external fragmentation.

Summary
Continuous memory allocation algorithms are fundamental to memory management. Choosing a strategy requires balancing allocation speed, fragmentation level, and system load. Understanding their principles aids in analyzing memory utilization issues and lays the foundation for learning non-contiguous allocation (e.g., paging).