Memory Management

Memory management is one of the most critical subsystems of an operating system. It handles the allocation, tracking, and回收 of primary memory (RAM), ensuring efficient utilization while providing process isolation and protection.

Why Memory Management Matters

Every program needs memory to store instructions, data, and stack frames. The OS must:

1. Allocate memory to processes when needed
2. Protect each process’s memory from others
3. Share memory efficiently when appropriate
4. Reclaim memory when processes terminate
5. Virtualize limited physical memory across many processes

Memory Hierarchy

graph TD
    A[CPU Registers] -->|~1 ns| B[L1 Cache]
    B -->|~2-4 ns| C[L2 Cache]
    C -->|~5-12 ns| D[L3 Cache]
    D -->|~50-100 ns| E[Main Memory - RAM]
    E -->|~5-10 ms| F[SSD / NVMe]
    F -->|~5-10 ms| G[HDD / Disk]
    
    style A fill:#ff6b6b,color:#fff
    style B fill:#ffa94d,color:#fff
    style C fill:#ffd43b,color:#000
    style D fill:#69db7c,color:#000
    style E fill:#4dabf7,color:#fff
    style F fill:#9775fa,color:#fff
    style G fill:#868e96,color:#fff

Address Translation

The fundamental challenge: programs use logical (virtual) addresses, but hardware needs physical addresses. The Memory Management Unit (MMU) translates between them.

graph LR
    A[CPU] -->|Virtual Address| B[MMU]
    B -->|Physical Address| C[Physical Memory]
    B -.->|TLB Hit| D[TLB Cache]
    D -.->|Fast Lookup| B
    
    style A fill:#4dabf7,color:#fff
    style B fill:#ff6b6b,color:#fff
    style C fill:#69db7c,color:#000
    style D fill:#ffa94d,color:#fff

Key Concepts Across This Section

Allocation Strategies

• Contiguous Allocation — Simplest approach; processes get consecutive physical blocks
• Paging — Fixed-size blocks; eliminates external fragmentation
• Segmentation — Variable-size segments matching program structure

Page Table Management

• Page Tables — Core data structures for address translation
• TLB — Hardware cache for page table entries
• Multi-Level Page Tables — Hierarchical tables to save space
• Inverted Page Tables — One entry per physical frame

Advanced Techniques

• Huge Pages — Larger page sizes for reduced TLB misses
• Swapping — Moving pages to/from disk
• mmap — Memory-mapped files and anonymous mappings
• NUMA — Non-Uniform Memory Access architectures

Allocator Implementations

• Allocation Algorithms — First-fit, best-fit, worst-fit
• Buddy System — Power-of-2 splitting/merging allocator
• Slab Allocator — Kernel object caching

Linux Memory Architecture

graph TB
    subgraph "User Space"
        A[Process A - Virtual Memory]
        B[Process B - Virtual Memory]
        C[Process C - Virtual Memory]
    end
    
    subgraph "Kernel Space"
        D[Virtual Memory Manager]
        E[Page Frame Allocator]
        F[Slab Allocator]
        G[Buddy System]
        H[Swap Manager]
    end
    
    subgraph "Hardware"
        I[MMU + TLB]
        J[Physical RAM - Page Frames]
        K[Swap Space - Disk]
    end
    
    A --> D
    B --> D
    C --> D
    D --> E
    D --> F
    D --> H
    E --> G
    F --> G
    G --> J
    H --> K
    I -.-> D
    I -.-> J
    
    style A fill:#4dabf7,color:#fff
    style B fill:#4dabf7,color:#fff
    style C fill:#4dabf7,color:#fff
    style D fill:#ff6b6b,color:#fff
    style J fill:#69db7c,color:#000
    style K fill:#868e96,color:#fff

Quick Reference: Key Terms

Interview Focus Areas

1. Paging vs Segmentation — trade-offs, why paging won
2. Page fault handling — step-by-step from trap to return
3. TLB — what happens on miss, TLB reach
4. Thrashing — causes, detection, solutions
5. Virtual to physical translation — walk through the hardware
6. Linux /proc/meminfo — understanding each field
7. malloc vs mmap — when the kernel uses each
8. Copy-on-Write — fork() optimization mechanics

Study Path

graph LR
    A[Contiguous] --> B[Paging]
    B --> C[Page Tables]
    C --> D[Multi-Level PT]
    B --> E[Segmentation]
    C --> F[TLB]
    D --> G[Huge Pages]
    B --> H[Demand Paging]
    H --> I[Page Replacement]
    H --> J[Thrashing]
    B --> K[mmap]
    B --> L[Swapping]
    
    style A fill:#4dabf7,color:#fff
    style B fill:#ff6b6b,color:#fff
    style H fill:#ffa94d,color:#fff

Start with contiguous allocation (simplest), then paging (modern standard), then build up to advanced topics.

Cross References

• Virtual Memory
• Cache Hierarchy
• Buffer Pool
• DRAM