Memory Management in Operating Systems

Definition:
- Memory management is the process by which an operating system controls and coordinates computer memory, allocating it to various running processes as needed.
Functions of Memory Management:
- Allocation: Assigning portions of memory to various processes.
- Deallocation: Reclaiming memory that is no longer in use.
- Protection: Ensuring that processes do not interfere with each other’s memory.
- Swapping: Moving data between main memory and disk when necessary.
Key Concepts:
- Processes and Memory: Each process is allocated its own address space to prevent interference.
- Virtual Memory: A technique that gives the illusion of a large key memory space, using disk storage as an extension of RAM.
- Physical Memory: The actual RAM installed in the computer.
Memory Allocation Strategies:
- Contiguous Allocation: Allocating a single contiguous block of memory to a process.
- Paging: Dividing memory into fixed-size blocks (pages) to eliminate external fragmentation.
- Segmentation: Dividing memory into variable-size segments based on the logical structure of processes.
Paging Mechanism:
- Page Table: Data structure used to store the mapping between logical addresses (used by programs) and physical addresses (used by the memory).
- Page Fault: Occurs when a program accesses a page not currently in memory, triggering the OS to fetch it from disk.
Swapping:
- Swap Space: A portion of the disk used for swapping pages in and out of physical memory.
- Thrashing: Excessive swapping resulting from inadequate memory, leading to a drastic decrease in performance.
Fragmentation:
- External Fragmentation: Occurs when free memory is split into small blocks scattered throughout, making it difficult to allocate large contiguous blocks.
- Internal Fragmentation: Occurs when allocated memory blocks are larger than required, wasting space.
Memory Management Algorithms:
- First Fit: Allocates the first block of memory that is large enough.
- Best Fit: Allocates the smallest available block that can satisfy the request, minimizing wasted space.
- Worst Fit: Allocates the largest available block to leave as many large blocks free as possible.
Performance Metrics:
- Hit Ratio: The ratio of accesses to the memory that are successfully completed without a page fault.
- Throughput: The number of processes completed in a given amount of time.
- Latency: The time taken to fulfill a memory access request.
Memory Management Units (MMUs):
- Hardware components that handle all memory access requests and translations of virtual addresses to physical addresses.

Memory Management

The operating system (OS) controls and manages computer memory by allocating it to running programs.
This process involves allocation (assigning memory to processes), deallocation (reclaiming unused memory), memory protection (ensuring processes don't interfere with each other's memory), and swapping (moving data between main memory and disk temporarily).
Each program has its own address space to prevent conflicts.
Virtual memory allows programs to access more space than physically available by using disk storage as an extension of RAM.
Physical memory refers to the actual RAM installed in the system.

Memory-Allocation Strategies

Contiguous Allocation: Allocating a continuous block of memory to each program, leading to external fragmentation when memory is fragmented.
Paging: Dividing memory into fixed-size pages to eliminate external fragmentation. Each page is mapped to a specific location in physical memory.
Segmentation: Dividing memory into variable-size segments based on the logical structures of programs, aiming to represent data and instructions properly.

Paging Mechanism

Page Table: A data structure that maps logical addresses used by programs to physical addresses used by memory.
Page Fault: Occurs when a program tries to access a specific page not currently in physical memory, triggering the OS to fetch it from disk.

Swapping

Swap Space: A designated area on the disk used to store pages that are temporarily not in physical memory during swapping.
Thrashing: Excessive swapping between memory and disk, significantly hindering system performance, due to a lack of available memory.

Fragmentation

External Fragmentation: Happens when free memory is split into small blocks scattered throughout, making it difficult to allocate a large contiguous block.
Internal Fragmentation: Occurs when allocated memory blocks are larger than required by the program, leading to unused space within the block.

Memory Management Algorithms

First Fit: The first available memory block that is large enough is allocated to the program.
Best Fit: Allocates the smallest available block that can fit the program's requirements, often minimizing wasted space.
Worst Fit: Allocates the largest available block, aiming to leave as many larger free blocks available for future requests.

Performance Metrics

Hit Ratio: The proportion of memory access requests successfully completed without encountering a page fault.
Throughput: The number of programs successfully completed within a given time period.
Latency: The time it takes to complete a memory access request.

Memory Management Units (MMUs)

Hardware Components that handle all memory access requests and translate logical addresses used by programs into physical addresses used by the system.