optimal page replacement algorithm in java

#Optimal page replacement algorithm in java software#
#Optimal page replacement algorithm in java free#

In addition, in most architectures the page table holds an 'access' bit and a 'dirty' bit for each page in the page table.

A page table maps a subset of the process virtual addresses to physical addresses. Each process has its own virtual address space. Modern general purpose computers and some embedded processors have support for virtual memory. Detecting which pages are referenced and modified However global page replacement is more efficient on an overall system basis. The advantage of local page replacement is its scalability: each process can handle its page faults independently, leading to more consistent performance for that process. Most popular forms of partitioning are fixed partitioning and balanced set algorithms based on the working set model. Local page replacement assumes some form of memory partitioning that determines how many pages are to be assigned to a given process or a group of processes.

#Optimal page replacement algorithm in java free#

When a process incurs a page fault, a local page replacement algorithm selects for replacement some page that belongs to that same process (or a group of processes sharing a memory partition).Ī global replacement algorithm is free to select any page in memory. Replacement algorithms can be local or global. As a result, page replacement in modern kernels ( Linux, FreeBSD, and Solaris) tends to work at the level of a general purpose kernel memory allocator, rather than at the higher level of a virtual memory subsystem. Moreover, as the goal of page replacement is to minimize total time waiting for memory, it has to take into account memory requirements imposed by other kernel sub-systems that allocate memory. For example, they can be locked, or can have write ordering requirements imposed by journaling. The latter pages have specific properties. In particular, most modern OS kernels have unified virtual memory and file system caches, requiring the page replacement algorithm to select a page from among the pages of both user program virtual address spaces and cached files. Requirements for page replacement algorithms have changed due to differences in operating system kernel architectures. This is mostly attributed to the spread of object-oriented programming techniques that favor large numbers of small functions, use of sophisticated data structures like trees and hash tables that tend to result in chaotic memory reference patterns, and the advent of garbage collection that drastically changed memory access behavior of applications.

#Optimal page replacement algorithm in java software#

Locality of reference of user software has weakened.

The cost of a CPU cache miss is far more expensive. With several gigabytes of primary memory, algorithms that require a periodic check of each and every memory frame are becoming less and less practical.

Size of primary storage has increased by multiple orders of magnitude.

In particular, the following trends in the behavior of underlying hardware and user-level software have affected the performance of page replacement algorithms: Since then, some basic assumptions made by the traditional page replacement algorithms were invalidated, resulting in a revival of research. That mostly ended with the development of sophisticated LRU (least recently used) approximations and working set algorithms. Page replacement algorithms were a hot topic of research and debate in the 1960s and 1970s.

10.1 Techniques for hardware with no reference bit.

9.10 Longest distance first (LDF) page replacement algorithm.

9.1 The theoretically optimal page replacement algorithm.

3 Detecting which pages are referenced and modified.

The page replacing problem is a typical online problem from the competitive analysis perspective in the sense that the optimal deterministic algorithm is known. A page replacement algorithm looks at the limited information about accesses to the pages provided by hardware, and tries to guess which pages should be replaced to minimize the total number of page misses, while balancing this with the costs (primary storage and processor time) of the algorithm itself. This determines the quality of the page replacement algorithm: the less time waiting for page-ins, the better the algorithm. When the page that was selected for replacement and paged out is referenced again it has to be paged in (read in from disk), and this involves waiting for I/O completion. Page replacement happens when a requested page is not in memory ( page fault) and a free page cannot be used to satisfy the allocation, either because there are none, or because the number of free pages is lower than some threshold. In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out, sometimes called swap out, or write to disk, when a page of memory needs to be allocated. processor, disk, database, web), see Cache algorithms. For an outline of general cache algorithms (e.g. This article is about algorithms specific to paging.