Internal vs. External Fragmentation in Operating Systems
Fragmentation is a phenomenon that occurs in memory management, where the system's memory space becomes inefficiently utilized. It can be broadly classified into two types: Internal Fragmentation and External Fragmentation. Both of these lead to wasted memory, but they occur in different ways and require different methods of resolution.
1. Internal Fragmentation:
Internal Fragmentation refers to the wasted space within allocated memory blocks. This happens when the memory allocated to a process is larger than the memory it actually needs. The difference between the allocated memory and the used memory becomes unused and unavailable for other processes.
How It Happens:
- When memory is divided into fixed-size blocks (e.g., pages or segments), a process may not fully utilize the allocated block.
- The unused portion of the block is wasted.
- This happens because the system can only allocate memory in chunks of a specific size, and if the process doesn’t require all the space, the leftover space within the allocated block cannot be used by other processes.
Example of Internal Fragmentation:
- If a system allocates memory in 4 KB blocks and a process requires only 2 KB, the remaining 2 KB in the block cannot be used by another process. This leads to internal fragmentation.
Advantages:
- Simpler Memory Allocation: It occurs in systems using fixed-size memory allocation schemes, like paging, where memory is divided into uniform blocks.
Disadvantages:
- Wasted Memory: The unused portion within allocated blocks can lead to inefficient use of memory.
- Decreased Efficiency: Over time, as processes continue to be allocated memory, the internal fragmentation can accumulate, leading to a significant amount of wasted space.
2. External Fragmentation:
External Fragmentation occurs when free memory is scattered throughout the system in small chunks. Even though there may be enough total free memory available to satisfy a process's request, it is non-contiguous and thus, the memory cannot be allocated in one contiguous block.
How It Happens:
- When processes are allocated and deallocated memory in variable-sized chunks (like in dynamic memory allocation), free memory becomes fragmented into many small blocks scattered throughout the system.
- If a new process requests a block of memory larger than any of the available free blocks (even though their total sum is enough), the allocation cannot be fulfilled due to the lack of contiguous free memory.
Example of External Fragmentation:
- Suppose a system has free memory blocks of size 2 KB, 3 KB, and 1 KB, but a process requires 4 KB of memory. Even though the total free memory is 6 KB, no single block of 4 KB is available, leading to external fragmentation.
Advantages:
- Flexible Memory Allocation: It occurs in systems using dynamic memory allocation schemes like variable partitioning, where processes can be allocated exactly the memory they need.
Disadvantages:
- Wasted Space: Though there may be enough total free memory, it is scattered and non-contiguous, leading to inefficiency.
- Complicated Memory Management: Managing external fragmentation typically requires techniques like compaction or paging.
Differences Between Internal and External Fragmentation:
| Feature | Internal Fragmentation | External Fragmentation |
|---|---|---|
| Definition | Wasted memory within allocated blocks. | Wasted memory due to small, scattered free memory blocks. |
| Occurs In | Fixed-size memory allocation (e.g., paging). | Variable-size memory allocation (e.g., dynamic partitioning). |
| Memory Wasted | Wasted within the allocated space. | Wasted between allocated spaces. |
| Solution | Reducing block size or using more efficient memory allocation techniques. | Compaction, paging, segmentation. |
| Example | A 4 KB block allocated for a process that uses only 2 KB. | Free spaces of 2 KB, 3 KB, and 1 KB, but no single block of 4 KB. |
Solutions to Fragmentation:
For Internal Fragmentation:
- Smaller Allocation Units: Decrease the size of the fixed memory block, but this may introduce more overhead.
- Paging or Segmentation: In paging, memory is divided into small fixed-size blocks (pages), and in segmentation, memory is divided based on logical segments.
For External Fragmentation:
- Compaction: This involves moving allocated processes in memory to create larger contiguous blocks of free memory.
- Paging/Segmentation: These methods break memory into small chunks or logical segments to avoid fragmentation.
- Memory Pooling: Allocation of memory in larger pools can help reduce fragmentation.
Conclusion:
- Internal Fragmentation occurs when memory blocks are allocated in fixed sizes, leading to wasted space inside allocated blocks.
- External Fragmentation arises when free memory is scattered across non-contiguous blocks, causing issues with allocating larger chunks of memory.
- Both types of fragmentation lead to inefficient memory use, but they require different solutions and approaches to manage effectively.
