5.4 Virtual memory: paging and segmentation

Virtual memory is a game-changer in computer architecture. It lets your computer run more apps than it has physical memory for by swapping data between RAM and disk. This clever trick gives each process its own virtual address space, keeping things safe and efficient.

Paging and segmentation are two ways to make virtual memory work. Paging splits memory into fixed-size chunks, while segmentation uses variable-sized logical units. Both help manage memory, but paging is more common in modern systems due to its simplicity.

Virtual Memory: Concept and Role

Memory Management Technique

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• Virtual memory is a memory management technique that allows a computer to compensate for shortages of physical memory by temporarily transferring data from RAM to disk storage
• Enables a computer to run more applications and processes simultaneously, even if the total memory requirements exceed the available physical RAM
• The operating system manages virtual memory by creating a virtual address space for each process, which is divided into fixed-size pages or variable-size segments
• The memory management unit (MMU) is responsible for translating virtual addresses to physical addresses and handling page faults when required data is not found in physical memory

Benefits and Advantages

• Allows running multiple applications concurrently, even when their combined memory requirements surpass the physical RAM capacity
• Provides each process with its own isolated virtual address space, preventing unauthorized access between processes
• Enables efficient utilization of available physical memory by allocating and deallocating pages or segments as needed
• Supports memory protection and access control mechanisms, enhancing system stability and security

Address Translation and Page Tables

Address Translation Process

• Address translation is the process of converting a virtual address used by a program into a corresponding physical address in RAM
• In a paging system, the virtual address space is divided into fixed-size pages, and the physical memory is divided into frames of the same size
• The MMU uses the page number from the virtual address as an index into the page table to find the corresponding physical frame number
• If the required page is not found in physical memory (a page fault occurs), the operating system intervenes to load the page from disk into an available frame and updates the page table accordingly

Page Table Structure and Usage

• Page tables are used to map virtual page numbers to physical frame numbers, enabling the MMU to perform address translation
• The page table contains entries for each virtual page, including the corresponding physical frame number and additional information such as valid/invalid bits and access permissions
• The operating system maintains and updates the page table to reflect the current mapping between virtual and physical memory
• Page table entries may also include additional flags or bits to indicate page status, such as dirty (modified) or referenced (recently accessed) bits

Paging vs Segmentation

Paging Characteristics

• Paging divides the virtual address space into fixed-size pages and the physical memory into frames of the same size
• Simplifies memory management by using a single page table and fixed-size pages, making it easier to allocate and deallocate memory
• Can suffer from internal fragmentation when a process does not fully utilize its allocated pages, wasting memory space
• Widely used in modern operating systems due to its simplicity and hardware support (x86, ARM)

Segmentation Characteristics

• Segmentation divides the virtual address space into variable-size segments based on logical units such as functions or data structures
• Allows for better memory utilization by allocating memory based on the actual size of logical units, reducing internal fragmentation
• Requires more complex memory management, as each segment needs its own base address and limit, and the operating system must manage multiple segment tables
• Provides a more natural and flexible way to organize and protect memory based on logical program structure (code, data, stack segments)
• Less commonly used in modern operating systems compared to paging

Virtual Memory Performance: Page Faults and Thrashing

Page Fault Handling

• Page faults occur when a required page is not found in physical memory, causing the operating system to load the page from disk, which is a slow operation compared to accessing RAM
• The operating system handles page faults by locating the required page on disk, loading it into an available physical frame, updating the page table, and resuming the interrupted process
• Frequent page faults can significantly degrade system performance, as the CPU must wait for the required data to be loaded from disk
• Techniques such as prefetching, caching, and optimizing page replacement algorithms can help reduce the impact of page faults

Thrashing and Mitigation Strategies

• Thrashing occurs when a system spends more time swapping pages in and out of memory than executing useful work, leading to severe performance degradation
• Happens when the total working set of all processes exceeds the available physical memory, causing constant page faults and disk I/O
• To mitigate thrashing, operating systems employ techniques such as:
  • Working set management: Monitoring and adjusting the set of pages a process actively uses
  • Page replacement algorithms: Selecting which pages to evict from memory when a page fault occurs (LRU, LFU, Clock)
  • Memory ballooning: Dynamically adjusting the memory allocation of virtual machines in a hypervisor environment
• Proper sizing of physical memory and optimizing application memory usage can help reduce the occurrence of page faults and thrashing