Memory Allocation in Operating Systems

Study Notes

Memory Allocation in Operating Systems

Definition: Memory allocation refers to the process of assigning portions of memory to various programs and data structures as needed during their execution.
Types of Memory Allocation:
1. Static Allocation:
  - Memory is allocated at compile time.
  - Size of memory must be known beforehand.
  - Examples: global variables, static variables.
2. Dynamic Allocation:
  - Memory is allocated at runtime.
  - Uses functions like malloc, calloc, realloc, and free in languages like C.
  - Allows for flexible memory usage based on program needs.
Memory Management Techniques:
1. Contiguous Memory Allocation:
  - Each process is allocated a single contiguous block of memory.
  - Simplicity but can lead to fragmentation.
2. Paging:
  - Divides memory into fixed-size pages.
  - Allows non-contiguous allocation, reducing fragmentation.
  - Uses a page table to map virtual addresses to physical addresses.
3. Segmentation:
  - Divides memory into variable-sized segments based on logical divisions (e.g., functions, data).
  - Each segment has a segment table for mapping.
4. Virtual Memory:
  - Extends physical memory onto disk storage.
  - Allows running larger applications than the physical memory can hold.
  - Uses techniques like paging and segmentation to manage memory.
Fragmentation:
- Internal Fragmentation: Wasted space within allocated memory blocks.
- External Fragmentation: Free memory is scattered in small pieces, making it difficult to allocate larger chunks.
Allocation Strategies:
1. First Fit: Allocates the first block of memory that is large enough.
2. Best Fit: Allocates the smallest block that fits the requirement to minimize wasted space.
3. Worst Fit: Allocates the largest available block, aiming to leave larger remaining free blocks.
Garbage Collection:
- Automatic memory management technique that frees memory occupied by objects no longer in use.
- Reduces memory leaks and fragmentation.
- Common in languages like Java and Python.
Key Concepts:
- Heap: Area of memory used for dynamic allocation.
- Stack: Area of memory for function calls and local variables, follows LIFO (Last In, First Out) principle.
Performance Considerations:
- Allocation speed and fragmentation impact overall system performance.
- Efficient memory allocation strategies are crucial for resource management in multitasking environments.

Memory Allocation in Operating Systems

Memory Allocation: The assignment of memory portions to programs and data structures during execution.

Types of Memory Allocation

Static Allocation:
- Allocated during compile time.
- Requires prior knowledge of memory size.
- Commonly used for global and static variables.
Dynamic Allocation:
- Allocated during runtime.
- Utilizes functions such as malloc, calloc, realloc, and free in C.
- Provides flexibility to adjust memory needs based on program requirements.

Memory Management Techniques

Contiguous Memory Allocation:
- Allocates a single contiguous block for each process.
- Offers simplicity but risks fragmentation.
Paging:
- Divides memory into fixed-size pages.
- Allows non-contiguous memory allocation, minimizing fragmentation impact.
- Utilizes a page table for virtual to physical address mapping.
Segmentation:
- Segments memory into variable-sized portions based on logical structures (e.g., functions).
- Each segment is tracked by a segment table.
Virtual Memory:
- Expands physical memory capability by using disk storage.
- Enables running applications larger than physical memory capacity.
- Combines paging and segmentation for effective management.

Fragmentation

Internal Fragmentation: Occurs when allocated memory blocks contain unused space.
External Fragmentation: Results from scattered free memory blocks, complicating allocation of larger contiguous spaces.

Allocation Strategies

First Fit: Allocates the first sufficient memory block found.
Best Fit: Chooses the smallest adequate block to optimize space usage.
Worst Fit: Selects the largest available block to retain more sizable free blocks.

Garbage Collection

An automatic mechanism to reclaim memory from objects that are no longer in use.
Aims to reduce memory leaks and fragmentation.
Widely employed in languages like Java and Python.

Key Concepts

Heap: Memory segment reserved for dynamic allocation.
Stack: Memory area designated for function calls and local variables; operates on a Last In, First Out (LIFO) basis.

Performance Considerations

Allocation speed and fragmentation significantly influence overall system performance.
Effective memory allocation strategies are essential for optimal resource management, particularly in multitasking scenarios.

Description

This quiz covers the concepts of memory allocation in operating systems, including static and dynamic memory allocation methods. It also explores various memory management techniques such as contiguous memory allocation and paging. Test your knowledge on how memory is managed during program execution.