Internal vs external fragmentation in Operating Systems - Key Differences Explained

Introduction

Imagine trying to fit different sized boxes into a storage space. Sometimes, even if there is enough total space, you can't use it efficiently because of how the space is divided. This problem happens in computers too, when managing memory.

Explanation

Internal Fragmentation

Internal fragmentation happens when memory is divided into fixed-size blocks, and a process does not use all the space in its assigned block. The leftover space inside that block remains unused but cannot be given to other processes. This wastes memory inside allocated blocks.

Internal fragmentation wastes memory inside allocated blocks due to fixed block sizes.

External Fragmentation

External fragmentation occurs when free memory is split into small scattered pieces over time. Even if the total free memory is enough for a process, it cannot be allocated because the free pieces are not contiguous. This wastes memory outside allocated blocks.

External fragmentation wastes memory outside allocated blocks due to scattered free spaces.

Real World Analogy

Imagine a parking lot with fixed-size parking spots. If a small car parks in a large spot, the extra space in that spot is wasted (internal fragmentation). Over time, if cars park and leave randomly, empty spots may be scattered, making it hard to find a big enough space for a large vehicle even if enough total space exists (external fragmentation).

Internal Fragmentation → Small car parked in a large fixed-size parking spot leaving unused space inside the spot

External Fragmentation → Empty parking spots scattered around the lot making it hard to find a big enough continuous space

Diagram

Diagram

┌───────────────────────────────┐
│ Allocated Blocks (Fixed Size) │
├─────────────┬─────────────┬───┤
│ Block 1     │ Block 2     │   │
│ Used 80%    │ Used 50%    │   │
│ Unused 20% │ Unused 50%  │   │
└─────────────┴─────────────┴───┘

Free Memory:
┌───┬─────┬───┬─────┬───┐
│ F │ F   │ F │ F   │ F │
│ r │ r   │ r │ r   │ r │
│ a │ a   │ a │ a   │ a │
│ g │ g   │ g │ g   │ g │
│ m │ m   │ m │ m   │ m │
│ e │ e   │ e │ e   │ e │
│ n │ n   │ n │ n   │ n │
│ t │ t   │ t │ t   │ t │
│ s │ s   │ s │ s   │ s │
└───┴─────┴───┴─────┴───┘

Diagram shows fixed-size allocated blocks with unused space inside (internal fragmentation) and scattered free memory fragments (external fragmentation).

Key Facts

Internal Fragmentation → Unused memory inside allocated fixed-size blocks due to allocation size mismatch.

External Fragmentation → Unused memory outside allocated blocks caused by scattered free spaces.

Fixed-size Memory Blocks → Memory divided into equal-sized chunks for allocation.

Contiguous Memory → Memory blocks placed next to each other without gaps.

Common Confusions

Believing internal fragmentation means free memory is scattered.

Believing internal fragmentation means free memory is scattered. Internal fragmentation is about wasted space inside allocated blocks, not scattered free memory.

Thinking external fragmentation happens inside allocated blocks.

Thinking external fragmentation happens inside allocated blocks. External fragmentation happens in free memory outside allocated blocks, not inside them.

Summary

Internal fragmentation wastes memory inside fixed-size allocated blocks when the process uses less than the block size.

External fragmentation wastes memory outside allocated blocks due to scattered free spaces that are too small to use.

Both types of fragmentation reduce efficient memory use but happen in different ways.