What Is External Fragmentation?

External fragmentation occurs when free memory is divided into scattered blocks, making it difficult to allocate contiguous space for new processes.

What Is Meant by External Fragmentation?

External fragmentation is a phenomenon in memory management where free memory is available but distributed across numerous non-contiguous blocks, making it challenging to allocate a single large block of memory to a process. This occurs because, over time, as processes are loaded and removed from memory, small gaps of unusable memory accumulate between allocated regions.

These gaps may not be large enough to meet the requirements of new processes, even if the total free memory is sufficient in size. External fragmentation is particularly problematic in systems that rely on dynamic memory allocation, as it can lead to inefficient use of memory resources and necessitate strategies such as compaction or paging to mitigate its effects.

External Fragmentation Example

Consider a system with 100 MB of free memory. Initially, memory is allocated as follows:

1. Process A: 10 MB
2. Process B: 20 MB
3. Process C: 30 MB

After some time, Processes A and C terminate, freeing their memory. The memory layout now looks like this:

• Free block: 10 MB
• Occupied block: 20 MB (Process B)
• Free block: 30 MB

Now, a new process, Process D, requires 35 MB of memory. Although the total free memory (10 MB + 30 MB = 40 MB) exceeds the requirement, Process D cannot be allocated because there is no single contiguous block of 35 MB.

This situation demonstrates external fragmentation, where free memory is available but scattered across non-contiguous spaces, preventing efficient allocation.

What Causes External Fragmentation?

External fragmentation is primarily caused by the dynamic allocation and deallocation of memory blocks in varying sizes. Over time, as processes request memory and then release it, gaps or holes are left in memory that are often too small to be reused efficiently. The key causes of external fragmentation include:

• Non-contiguous memory allocation. Memory blocks are allocated based on availability, not continuity, leading to scattered free spaces.
• Variable-sized memory requests. Different processes require different amounts of memory. When smaller processes are allocated in spaces left by larger ones, uneven gaps are created.
• Process termination or deallocation. When processes end or release memory, the freed spaces may not align with future requests, leaving behind unusable memory segments.
• First-fit or best-fit allocation strategies. Memory allocation methods like first-fit (assigning the first available block) and best-fit (assigning the smallest suitable block) can contribute to fragmentation by leaving small, unusable gaps between allocated blocks.
• Long-running systems. Over time, systems that handle a large number of allocation and deallocation cycles accumulate fragmented memory as a result of repeated operations.

What Are the Effects of External Fragmentation?

External fragmentation can have several significant effects on system performance and memory utilization, including:

• Inefficient memory usage. Even though sufficient total memory might be available, it may be unusable for processes that require larger contiguous blocks, leading to underutilization of resources.
• Reduced system performance. Fragmentation increases the time spent searching for suitable memory blocks during allocation, potentially slowing down system operations.
• Allocation failures. Processes that require large contiguous memory blocks may fail to allocate, even if the combined free memory is sufficient.
• Frequent out-of-memory errors. As fragmentation accumulates, the system may erroneously report memory shortages due to insufficient contiguous space, even when free memory is available.
• Need for frequent memory compaction. To reclaim fragmented memory, systems may require compaction, a process of rearranging memory to consolidate free spaces, which can disrupt running processes and consume processing time.
• Wasted resources. Small, scattered free blocks often remain unused, leading to a scenario where physical memory is available but cannot be effectively allocated to fulfill process requirements.

How to Prevent External Fragmentation?

Preventing external fragmentation involves implementing memory management techniques that minimize or eliminate the need for contiguous allocation of memory. Common strategies include:

• Paging. Memory is divided into fixed-sized blocks called pages, and processes are allocated pages rather than contiguous memory. This eliminates the need for contiguous allocation, effectively avoiding external fragmentation.
• Segmentation with paging. A combination of segmentation and paging maps logical memory segments into fixed-sized pages, reducing fragmentation while maintaining flexibility in memory usage.
• Buddy memory allocation. This system allocates memory in blocks of sizes that are powers of two, splitting and merging blocks as needed. This minimizes fragmentation by ensuring that memory is allocated in predictable sizes.
• Memory compaction. Periodically reorganizing memory by moving allocated blocks together and consolidating free spaces can reduce fragmentation. However, this approach is resource-intensive and may disrupt system performance.
• Using best-fit or worst-fit allocation wisely. Allocation strategies can be tailored to minimize fragmentation. For instance, best-fit may leave smaller gaps, while worst-fit might ensure larger gaps that are reusable for other processes.
• Dynamic memory management improvements. Advanced algorithms can predict allocation patterns and optimize memory usage to reduce fragmentation over time.
• Using virtual memory. Virtual memory decouples logical memory from physical memory, allowing processes to access memory without concern for its physical layout, thereby avoiding fragmentation.

How to Fix External Fragmentation?

Fixing external fragmentation typically involves reorganizing or rethinking how memory is allocated and managed. Common methods include:

• Memory compaction. Rearrange memory to consolidate scattered free blocks into a single contiguous block. While effective, this process is computationally expensive and may disrupt running processes, so it is often used sparingly.
• Dynamic memory defragmentation. Employ algorithms that actively shuffle memory allocations in the background to minimize fragmentation without pausing system operations.
• Switch to paging. Replace contiguous memory allocation with paging. In this approach, memory is divided into fixed-sized blocks (pages), and processes can use non-contiguous pages, eliminating external fragmentation altogether.
• Segmentation with paging. Combine segmentation and paging to provide logical separation for processes while mapping to physical pages, avoiding the need for contiguous memory blocks.
• Memory allocation strategies. Reevaluate allocation methods such as first-fit, best-fit, or worst-fit. Some methods may exacerbate fragmentation, while others can leave larger reusable blocks of memory.
• Memory reclamation. Implement garbage collection systems to free up memory held by terminated processes or unused objects, ensuring fragmented spaces are reclaimed for future use.
• Using virtual memory. Virtual memory allows the operating system to simulate larger contiguous memory spaces by mapping virtual addresses to physical ones, effectively bypassing physical fragmentation.

Advantages and Disadvantages of External Fragmentation

Here are the main advantages and disadvantages of external fragmentation.

What Are the Advantages of External Fragmentation?

While external fragmentation is generally considered a drawback in memory management, it can occasionally have benefits depending on the system’s needs and memory allocation strategies. These advantages arise when flexibility in memory usage and allocation is prioritized over strict efficiency.

1. Flexibility in Dynamic Memory Allocation

External fragmentation results from the use of dynamic memory allocation, which allows processes to request memory based on their specific needs. This flexibility ensures that memory is allocated only when required and in the exact size needed, reducing unnecessary over-allocation. While this can lead to dispersed free spaces, it also means that memory resources are utilized more precisely, accommodating diverse workloads.

2. Better Suitability for Large and Diverse Processes

In systems where processes vary widely in memory requirements, external fragmentation allows these diverse processes to be accommodated without enforcing strict uniformity in allocation sizes. This adaptability ensures that large or unique processes can still find sufficient memory, even if the layout is irregular, as long as a contiguous block of the required size is available.

3. Reduced Overhead Compared to Compaction

Unlike compaction or paging systems, which require additional processing or hardware support to manage memory, external fragmentation occurs naturally as a byproduct of simple allocation strategies. This reduces system overhead in terms of complexity and resource consumption, making it a trade-off that can benefit systems where performance constraints outweigh the need for highly efficient memory utilization.

What Are the Disadvantages of External Fragmentation?

External fragmentation poses significant challenges to memory management, leading to inefficiencies and potential performance bottlenecks in systems relying on dynamic memory allocation. Below are the key disadvantages of external fragmentation.

1. Inefficient Memory Utilization

External fragmentation leaves memory divided into small, scattered, free blocks that are often too small to be used effectively. Even when sufficient total memory is available, the lack of contiguous space prevents its allocation to processes that require larger blocks. This inefficient utilization of memory can cause resources to be wasted, reducing the system's overall operational capacity.

2. Allocation Failures Despite Available Memory

A critical drawback of external fragmentation is that it can lead to allocation failures, even when total free memory exceeds a process's requirements. For example, if a process requests 50 MB of memory but the largest contiguous block available is only 40 MB, the allocation fails, resulting in inefficiencies and possible interruptions to operations.

3. Increased System Overhead

The presence of fragmented memory increases the computational overhead for memory allocation. Searching for suitable blocks that fit the process requirements takes longer in a fragmented memory layout. This additional processing time degrades the performance of applications and slows down system operations.

4. Need for Frequent Memory Compaction

Addressing external fragmentation often requires memory compaction, where free spaces are consolidated into contiguous blocks. While effective, this process is time-consuming and computationally expensive, and it may temporarily disrupt running processes. Frequent compaction adds to system overhead and affects user experience, especially in real-time applications.

5. Scalability Limitations in Long-running Systems

Over time, external fragmentation accumulates in systems with dynamic allocation, making them less capable of handling new processes efficiently. This issue becomes more pronounced in long-running systems or those with a high churn rate, where allocation and deallocation cycles are frequent. The resulting fragmentation restricts the scalability of the system, limiting its ability to support larger workloads.