Computer Science Memory Management

Memory Management

• Logical and Physical Address Space: A logical address is a reference to a memory location by the program, while a physical address is the actual memory location in RAM. These address spaces are usually different and need translation.

• Swapping: Swapping is a technique where inactive parts of a program in RAM are moved out to disk and replaced with other active parts, making space for other programs to be loaded and executed.

• Contiguous Allocation: Space for files on disk is blocked and assigned consecutively to the file itself

• Paging: Dividing the memory and files into fixed-size blocks called pages. The page table maps logical pages to physical frames.

• Structure of Page Table: A page table holds the mapping information between logical pages and physical frames

• Segmentation: Dividing the memory into variable-sized blocks called segments. This is suited to program structure because segments can have meaningful sizes and structures

• Segmentation with Paging: Combining segmentation's flexibility with paging's efficiency by segmenting the program and further breaking segments into pages

Virtual Memory

• Background: Virtual memory manages more memory than is physically available. It often uses secondary storage (disk) to create the illusion of a much larger RAM space.

• Demand Paging: Only loading pages into memory when they're required, improving efficiency and reducing the need for a large, continuously loaded RAM space.

• Performance of Demand Paging: Determined by factors like page fault rate and the speed of secondary storage. A lower page fault rate and faster secondary storage equate to better performance.

• Page Replacement: Necessary when demand for pages exceeds the available physical memory frames. This determines which pages in memory are removed.

• Page Replacement Algorithms: Different methods for choosing which pages to remove (e.g., FIFO, LRU, optimal). Each method has its trade-offs.

• Allocation of Frames: Determining how many memory frames are assigned to each process to optimize memory use and fairness between processes.

• Thrashing: A situation where frequently swapping pages in and out of memory causes significant overhead and seriously degrades system performance.

File System Interface and Implementation

• File System Interface: The way users and applications interact with file systems.

  • Concept of a File: A named collection of data, with attributes like size, permissions, and creation date.

  • Access Methods: Different ways of accessing data within a file (sequential, random, etc).

  • Directory Structure: Organizing files in a hierarchical tree or other structure to enable easy search & retrieval.

  • File System Mounting: Connecting a file system to a computer file system enabling the user to access data on attached devices, enabling access to files on external storage devices.

  • File Sharing: Enables multiple users to access and interact with files or directory (with access restrictions)

  • Protection: Maintaining access control to ensure files and system components are only used as intended.

• File System Implementation:

  • File System Structure: The internal organization of the file system, including methods for managing files and directories.
  • File Allocation Methods: How files are allocated space on the storage device (contiguous, linked, indexed).
  • Free-space Management: Tracking available space on storage devices to enable efficient file allocation.
  • Directory Implementation: How directories are structured and managed, including search and update algorithms for directories
  • Efficiency and Performance: Design decisions that try to improve the speed and response of the file system, such as caching and disk optimization.