First-Fit vs. Best-Fit Memory Allocation Algorithms: A Detailed Comparison

Compare and contrast the first-fit and best-fit memory allocation algorithms. This guide explains their mechanisms, evaluates their performance using examples, and discusses their trade-offs in terms of memory utilization, allocation speed, and external fragmentation.

Comparing First-Fit and Best-Fit Memory Allocation

Introduction: First-Fit and Best-Fit Algorithms

First-fit and best-fit are two common algorithms used in operating systems for allocating memory to processes. Both aim to allocate contiguous blocks of memory from a pool of available space, but they differ in how they select the memory block. Understanding these algorithms is crucial for appreciating how memory is managed within an operating system.

The Problem: Optimal Memory Allocation

Let's consider a scenario where memory partitions of sizes 25KB, 50KB, 100KB, and 75KB need to be allocated. The available memory has partitions of sizes 75KB, 50KB, 175KB, 200KB, and 150KB and are shown below:

Diagram of Memory Partitions

First-Fit Algorithm

The first-fit algorithm allocates the first available partition that's large enough to meet the process's requirements. It scans the memory partition list until it finds a free partition that can accommodate the request. Once a suitable block is found, it is allocated. The algorithm stops searching at the first successful allocation.

Applying First-Fit to the Example

  1. 25KB request: Allocates 25KB from the first 75KB partition (50KB remaining).
  2. 50KB request: Allocates the next 50KB partition (no remaining space).
  3. 100KB request: Allocates 100KB from the 175KB partition (75KB remaining).
  4. 75KB request: Allocates the remaining 75KB partition (no remaining space).

First-fit successfully allocates all requests optimally in this example.

Best-Fit Algorithm

The best-fit algorithm scans the entire list of available memory partitions and selects the smallest available partition that's large enough to satisfy the process's memory requirement. This aims to minimize wasted space (fragmentation) but requires more searching.

Applying Best-Fit to the Example

  1. 25KB request: Allocates 25KB from the 75KB partition (50KB remaining).
  2. 50KB request: Allocates the 50KB partition (no remaining space).
  3. 100KB request: Allocates 100KB from the 175KB partition (75KB remaining).
  4. 75KB request: Allocates 75KB from the remaining 150KB partition (75KB remaining).

Best-fit also allocates all requests optimally in this case.

Comparison: First-Fit vs. Best-Fit

In this example, both algorithms perform optimally. However, best-fit requires more searching (scanning the entire list each time), increasing overhead. First-fit is generally faster but might lead to more fragmentation in other scenarios.