Fixed Partition vs Variable Partition in Operating Systems, and the Need for Compaction

In memory partitioning, the operating system assigns each process to a contiguous region of RAM. Two classic contiguous allocation methods are fixed partitioning and variable partitioning. The essential difference is that fixed partitioning creates partitions in advance, while variable partitioning creates them on demand to better match process sizes.2

In fixed partitioning, main memory is split into a fixed number of partitions, each capable of holding at most one process. A process must fit entirely inside one available partition of equal or greater size. In variable partitioning, partitions are not predetermined; instead, the OS allocates a block approximately equal to the process's size when the process arrives, improving utilization.2

The trade-off is fragmentation. Fixed partitioning commonly suffers from internal fragmentation, because a process may be smaller than the partition assigned to it.2 Variable partitioning largely removes internal waste, but over time creates external fragmentation, where total free space may be sufficient yet no single contiguous block is large enough for a new process.2 This leads directly to the need for compaction, a technique that merges dispersed free holes into one larger usable region.2

Footnotes

1. Difference between Fixed Partitioning and Variable Partitioning - GeeksforGeeks - Overview of definitions, features, and comparison. ↩ ↩²

2. CSC 553 Operating Systems - Lecture 7 - Memory Management - Lecture notes explaining dynamic partitioning, compaction, and relocation cost. ↩ ↩²

3. Memory Management in Operating System - GeeksforGeeks - Summary of partitioned memory allocation and fixed partition characteristics. ↩ ↩²

4. Variable (or Dynamic) Partitioning in Operating System - GeeksforGeeks - Explanation of dynamic partitioning and its motivation. ↩ ↩²

5. Difference between Internal and External Fragmentation - GeeksforGeeks - Clarifies internal vs external fragmentation and the role of compaction. ↩ ↩²

6. Implementation of Contiguous Memory Management Techniques - GeeksforGeeks - Discusses fixed/variable partition schemes, placement algorithms, and fragmentation handling. ↩

Fixed Partitioning

In fixed partitioning, memory is divided into a predetermined number of static partitions during system setup or initialization.2 These partitions may be equal-sized or unequal-sized, but their count and boundaries do not change during execution. Since each partition holds only one process, the degree of multiprogramming is limited by the number of partitions.2

The chief advantage of this scheme is simplicity. The OS bookkeeping is relatively small, allocation is straightforward, and implementation overhead is low.2 However, it wastes memory when process sizes do not match partition sizes. If a $20\text{ KB}$ process is placed into a $32\text{ KB}$ partition, the unused $12\text{ KB}$ inside that partition is lost to internal fragmentation.2

A second limitation is process size restriction. If the largest partition is smaller than a process, that process cannot be loaded at all, even if total free memory across partitions is sufficient.2 Thus, fixed partitioning is simple but inflexible.

Footnotes

1. Difference between Fixed Partitioning and Variable Partitioning - GeeksforGeeks - Overview of definitions, features, and comparison. ↩

2. Memory Management in Operating System - GeeksforGeeks - Summary of partitioned memory allocation and fixed partition characteristics. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

3. Implementation of Contiguous Memory Management Techniques - GeeksforGeeks - Discusses fixed/variable partition schemes, placement algorithms, and fragmentation handling. ↩ ↩² ↩³

4. Difference between Internal and External Fragmentation - GeeksforGeeks - Clarifies internal vs external fragmentation and the role of compaction. ↩

Variable Partitioning

Variable partitioning, also called dynamic partitioning, was introduced to reduce the inefficiency of fixed partitions.2 Instead of pre-cutting memory into rigid blocks, the OS maintains free memory and allocates a block sized to the incoming process.2 Because the allocated region more closely matches the process size, internal fragmentation is greatly reduced or eliminated in the ideal model.2

This design improves memory utilization and supports a more flexible number of resident processes, because the number of partitions depends on the current workload rather than a static setup.2 It also avoids the fixed upper bound imposed by a predetermined partition count.

However, variable partitioning introduces more complex bookkeeping and placement decisions. The OS must decide where to place each new process using algorithms such as first fit, best fit, or next fit. Over time, as processes terminate and release memory, small holes form between allocated regions, producing external fragmentation.2

Footnotes

1. Difference between Fixed Partitioning and Variable Partitioning - GeeksforGeeks - Overview of definitions, features, and comparison. ↩ ↩² ↩³

2. Variable (or Dynamic) Partitioning in Operating System - GeeksforGeeks - Explanation of dynamic partitioning and its motivation. ↩ ↩² ↩³ ↩⁴

3. Implementation of Contiguous Memory Management Techniques - GeeksforGeeks - Discusses fixed/variable partition schemes, placement algorithms, and fragmentation handling. ↩ ↩² ↩³ ↩⁴

4. Difference between Internal and External Fragmentation - GeeksforGeeks - Clarifies internal vs external fragmentation and the role of compaction. ↩

Direct Difference Between Fixed and Variable Partitioning

The two methods differ across design, efficiency, and operational behavior:

A concise way to remember the distinction is:

$$\text{Fixed partitioning} \Rightarrow \text{simple but wasteful}$$ $$\text{Variable partitioning} \Rightarrow \text{efficient but fragmentation-prone}$$

Footnotes

1. Memory Management in Operating System - GeeksforGeeks - Summary of partitioned memory allocation and fixed partition characteristics. ↩

2. Memory Management - University lecture notes on fixed partitions, unequal partitions, dynamic partitioning, and compaction. ↩

3. Difference between Fixed Partitioning and Variable Partitioning - GeeksforGeeks - Overview of definitions, features, and comparison. ↩

4. Variable (or Dynamic) Partitioning in Operating System - GeeksforGeeks - Explanation of dynamic partitioning and its motivation. ↩

5. Difference between Internal and External Fragmentation - GeeksforGeeks - Clarifies internal vs external fragmentation and the role of compaction. ↩

6. CSC 553 Operating Systems - Lecture 7 - Memory Management - Lecture notes explaining dynamic partitioning, compaction, and relocation cost. ↩

7. Implementation of Contiguous Memory Management Techniques - GeeksforGeeks - Discusses fixed/variable partition schemes, placement algorithms, and fragmentation handling. ↩

Why Compaction Is Needed

Compaction is needed mainly in variable partitioning because free memory becomes fragmented into many small, separate holes after repeated loading and removal of processes.2 Suppose free memory exists as $10\text{ KB} + 12\text{ KB} + 18\text{ KB} = 40\text{ KB}$. A new process needing $30\text{ KB}$ still cannot be loaded if no single contiguous hole is at least $30\text{ KB}$. This is the hallmark of external fragmentation.

Compaction solves this by relocating allocated processes so that all free holes are merged into one larger contiguous block.2 After compaction, the same $40\text{ KB}$ total free space could become one $40\text{ KB}$ hole, allowing the $30\text{ KB}$ process to be admitted.2

Compaction therefore has three main purposes:

1. To overcome external fragmentation.2
2. To create larger contiguous blocks for incoming processes.2
3. To improve usable memory availability without physically increasing RAM.

Yet compaction is expensive. The OS must move processes in memory and update their addresses using relocation support.2 This consumes CPU time and can temporarily reduce system performance, so compaction is helpful but not free.2

Footnotes

1. CSC 553 Operating Systems - Lecture 7 - Memory Management - Lecture notes explaining dynamic partitioning, compaction, and relocation cost. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸

2. Implementation of Contiguous Memory Management Techniques - GeeksforGeeks - Discusses fixed/variable partition schemes, placement algorithms, and fragmentation handling. ↩ ↩² ↩³ ↩⁴ ↩⁵

3. Difference between Internal and External Fragmentation - GeeksforGeeks - Clarifies internal vs external fragmentation and the role of compaction. ↩ ↩² ↩³

Final Understanding

Fixed partitioning and variable partitioning are both contiguous allocation techniques, but they optimize different goals.2 Fixed partitioning prioritizes simplicity and low overhead, yet wastes memory because partition sizes are rigid. Variable partitioning improves fit and utilization by allocating memory according to process need, but this flexibility creates external fragmentation over time.2

The need for compaction arises because contiguous allocation requires a process to fit into one uninterrupted block of memory.2 When free space is scattered, total memory alone is not enough; its arrangement matters. Compaction restores usability by gathering fragmented free areas into a single contiguous region, although the relocation cost makes it computationally expensive.2

In short:

• Fixed partitioning $\rightarrow$ simple, static, more internal fragmentation.2
• Variable partitioning $\rightarrow$ flexible, dynamic, more external fragmentation.2
• Compaction $\rightarrow$ required to combat external fragmentation by merging free holes.3

Footnotes

1. Memory Management in Operating System - GeeksforGeeks - Summary of partitioned memory allocation and fixed partition characteristics. ↩ ↩² ↩³

2. Variable (or Dynamic) Partitioning in Operating System - GeeksforGeeks - Explanation of dynamic partitioning and its motivation. ↩ ↩² ↩³

3. Implementation of Contiguous Memory Management Techniques - GeeksforGeeks - Discusses fixed/variable partition schemes, placement algorithms, and fragmentation handling. ↩ ↩² ↩³

4. CSC 553 Operating Systems - Lecture 7 - Memory Management - Lecture notes explaining dynamic partitioning, compaction, and relocation cost. ↩ ↩² ↩³

5. Difference between Internal and External Fragmentation - GeeksforGeeks - Clarifies internal vs external fragmentation and the role of compaction. ↩ ↩²

6. Difference between Fixed Partitioning and Variable Partitioning - GeeksforGeeks - Overview of definitions, features, and comparison. ↩ ↩²