The Mastery of Computer Programming: Primary Algorithms - Sykalo Eugene 2023

Virtual memory
Memory Hierarchies and Optimization

Virtual Memory Concepts

Virtual memory is a technique used by operating systems to provide the illusion of a larger amount of memory than is physically available. It allows programs to access memory that is not directly available in main memory, which can improve performance by reducing the need to access slower secondary storage devices, such as hard drives.

The operating system plays a critical role in managing virtual memory. It is responsible for mapping virtual addresses to physical addresses, managing page tables, and determining when to move data between main memory and secondary storage.

Virtual memory addressing is a technique used to map virtual addresses to physical addresses. A virtual address is a reference to a memory location that is interpreted by the operating system, while a physical address is the actual location in memory. The translation from virtual address to physical address is performed by the Memory Management Unit (MMU).

Virtual memory systems use a variety of page replacement policies to manage the allocation of physical memory. These policies determine which pages are removed from main memory when space is needed. The Least Recently Used (LRU) algorithm is a commonly used page replacement policy that removes the page that has not been accessed for the longest period of time. Other algorithms used for page replacement include the Clock algorithm and the Random algorithm.

In summary, virtual memory is a technique used by operating systems to provide the illusion of a larger amount of memory than is physically available. Virtual memory systems use a variety of algorithms, such as page replacement policies, to manage the allocation of physical memory. By understanding these concepts, programmers can write code that takes advantage of the benefits of virtual memory and improves the performance of their applications.

Paging Algorithms

Paging is a technique used by virtual memory systems to manage the allocation of memory in fixed-size blocks called pages. When a process requests memory, the operating system assigns one or more pages to the process, which are then mapped to physical memory addresses by the Memory Management Unit (MMU).

Paging algorithms are used to manage the allocation and deallocation of pages in virtual memory systems. The most commonly used paging algorithm is the Least Recently Used (LRU) algorithm, which removes the page that has not been accessed for the longest period of time when a new page is requested. The LRU algorithm is effective because it takes advantage of the principle of locality, which states that recently accessed pages are more likely to be accessed again in the near future.

Other paging algorithms include the Clock algorithm and the Random algorithm. The Clock algorithm maintains a circular list of pages and uses a clock hand to determine which page to remove when space is needed. The Random algorithm selects a page to remove at random.

Paging algorithms can have a significant impact on the performance of virtual memory systems. The LRU algorithm is generally considered to be the most effective paging algorithm, although it can be expensive to implement in hardware. The Clock algorithm is a simpler alternative that is often used in practice.

In summary, paging algorithms are used to manage the allocation and deallocation of pages in virtual memory systems. The LRU algorithm is the most commonly used paging algorithm, although other algorithms such as the Clock algorithm and the Random algorithm are also used.

Segmentation Algorithms

Segmentation is a technique used by virtual memory systems to manage the allocation of memory in variable-size blocks called segments. Unlike paging, which divides memory into fixed-size pages, segmentation allows for more flexible allocation of memory based on the needs of the program.

Segmentation algorithms are used to manage the allocation and deallocation of segments in virtual memory systems. The Best Fit algorithm selects the smallest segment that is large enough to hold the requested memory, while the First Fit algorithm selects the first segment that is large enough to hold the requested memory. The Worst Fit algorithm selects the largest segment that is large enough to hold the requested memory.

Segmentation algorithms can have a significant impact on the performance of virtual memory systems. The Best Fit algorithm is generally considered to be the most effective segmentation algorithm because it minimizes fragmentation and maximizes the amount of usable memory. However, it can be expensive to implement in hardware. The First Fit algorithm is a simpler alternative that is often used in practice.

In summary, segmentation is a technique used by virtual memory systems to manage the allocation of memory in variable-size blocks called segments. Segmentation algorithms are used to manage the allocation and deallocation of segments in virtual memory systems, with the Best Fit algorithm generally considered to be the most effective algorithm.

Page Tables and Address Translation

Virtual memory systems use page tables to translate virtual addresses to physical addresses. A page table is a data structure that maps virtual addresses to physical addresses, allowing the Memory Management Unit (MMU) to translate virtual addresses used by programs into physical addresses used by the hardware.

The page table is typically stored in memory and is managed by the operating system. Each entry in the page table corresponds to a page of memory and contains information about the physical address of the page and its status (such as whether it is currently in main memory or on secondary storage).

The translation from a virtual address to a physical address is performed by the MMU using the page table. When a program accesses a virtual address, the MMU looks up the corresponding entry in the page table and uses the physical address stored in the entry to access the corresponding physical memory location.

The page table can be organized in a variety of ways to optimize performance. For example, some systems use a two-level page table, where the first level maps a portion of the virtual address to a second-level page table, which in turn maps the remaining portion of the virtual address to a physical address. This reduces the size of the page table and makes it faster to search.

Address translation can also be accelerated by using a Translation Lookaside Buffer (TLB), which is a cache of recently used page table entries. When an address is translated, the MMU first checks the TLB to see if the translation is already cached. If it is, the MMU can use the cached translation instead of performing a full page table lookup.

In summary, page tables are used by virtual memory systems to translate virtual addresses to physical addresses. The page table contains a mapping of virtual addresses to physical addresses, which is used by the Memory Management Unit (MMU) to translate virtual addresses into physical addresses. The page table can be optimized for performance by using techniques such as two-level page tables and Translation Lookaside Buffers (TLBs).

Demand Paging and Page Faults

Demand paging is a technique used by virtual memory systems to avoid loading all pages into main memory at once. Instead, pages are loaded on demand as they are needed by the program. This can improve performance by reducing the amount of memory required to run a program and by reducing the amount of time spent transferring data between main memory and secondary storage.

When a program accesses a page that is not currently in main memory, a page fault occurs. The operating system then retrieves the page from secondary storage and loads it into main memory. The page fault handling algorithm determines which pages should be removed from main memory to make room for the new page. This algorithm can use a variety of techniques, such as the Least Recently Used (LRU) algorithm, to determine which pages to remove.

The Demand Paging algorithm is used to manage the loading of pages in demand paging systems. When a page fault occurs, the Demand Paging algorithm retrieves the requested page from secondary storage and loads it into main memory. If there is not enough space in main memory to hold the new page, the Demand Paging algorithm calls the page fault handling algorithm to remove one or more pages from main memory to make room for the new page.

Page faults can have a significant impact on the performance of virtual memory systems. To minimize the number of page faults, it is important to optimize the page replacement policy used by the page fault handling algorithm. The LRU algorithm is often used because it takes advantage of the principle of locality, which states that recently accessed pages are more likely to be accessed again in the near future.

In summary, demand paging is a technique used by virtual memory systems to avoid loading all pages into main memory at once. When a program accesses a page that is not currently in main memory, a page fault occurs and the operating system retrieves the page from secondary storage and loads it into main memory. The Demand Paging algorithm is used to manage the loading of pages in demand paging systems, and the page fault handling algorithm is used to determine which pages to remove from main memory when space is needed.

Memory Management Unit (MMU)

The Memory Management Unit (MMU) is a hardware component responsible for managing virtual memory in a computer system. The MMU is responsible for translating virtual addresses used by programs into physical addresses used by the hardware, allowing programs to access memory that is not directly available in main memory.

When a program accesses a virtual address, the MMU looks up the corresponding entry in the page table and uses the physical address stored in the entry to access the corresponding physical memory location. The MMU also manages the Translation Lookaside Buffer (TLB), which is a cache of recently used page table entries. When an address is translated, the MMU first checks the TLB to see if the translation is already cached. If it is, the MMU can use the cached translation instead of performing a full page table lookup.

The MMU can be implemented in hardware or in software. Hardware implementations of the MMU are faster and more efficient, but are also more expensive to design and manufacture. Software implementations of the MMU are slower and less efficient, but are less expensive to implement and can be modified more easily.

The MMU can also be used to implement memory protection, which is a technique used to prevent programs from accessing memory that they are not authorized to access. Memory protection is implemented by assigning each page of memory a set of permissions that indicate whether the page can be read, written, or executed. The MMU checks these permissions before allowing a program to access a page of memory.

In summary, the Memory Management Unit (MMU) is a hardware component responsible for managing virtual memory in a computer system. The MMU translates virtual addresses used by programs into physical addresses used by the hardware, and manages the Translation Lookaside Buffer (TLB) to accelerate address translation. The MMU can be implemented in hardware or in software, and can also be used to implement memory protection to prevent programs from accessing unauthorized memory.