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Understanding Paging: Managing Virtual Memory and Physical RAM in Modern Systems

This document explores the concept of paging in computer systems, which addresses how processes utilize virtual memory when physical RAM is limited. It covers key topics such as the paging process, memory page management, and the role of the page file (pagefile.sys). It delves into important terms like working set, committed pages, and page faults, as well as common issues like thrashing and disk contention. Ultimately, strategies to optimize paging performance and system responsiveness are discussed to enhance overall computing efficiency.

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Understanding Paging: Managing Virtual Memory and Physical RAM in Modern Systems

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  1. T111 The Page File

  2. Last Week…

  3. Problem • What if the process or processes request more pages of virtual memory than there are pages of physical RAM? • The answer - copy the contents of a page of memory to some other storage device and then let that page of memory be used by a process

  4. Paging • When a process needs to access a page of memory: • Accesses the memory page in RAM • OR • The memory page is copied from the page file into RAM and then accessed • Page out – Memory page is moved from RAM to the page file • Page in – Memory page is moved from the page file back into RAM

  5. Virtual address space(per process) Paging Physical RAM Secondary storage Memory Page

  6. Where is the page file? • pagefile.sys • On the C:\ drive by default C:\>dir /ah pagefile.sys Volume in drive C is System_Disk Volume Serial Number is E087-4551 Directory of C:\ 08/09/2007 05:04 p.m. 1,598,029,824 pagefile.sys 1 File(s) 1,598,029,824 bytes 0 Dir(s) 5,069,359,104 bytes free

  7. Working Set • The active pages of virtual memory that a process uses during its execution • Only the active pages of virtual memory associated with a process will remain resident in physical RAM • The non-active pages will eventually be paged-out to secondary storage

  8. Committed Pages • A page of virtual memory that has been addressed by a process • They can either be in physical memory or in the page file • A committed page includes the active pages from the working set and the non-active pages that may have been paged-out to secondary storage

  9. Page Fault • When a process attempts to access a page of memory in RAM but that page is actually in the page file • The page will have to be ‘paged in’ before the process can access it • What will the process do while waiting for the page to be paged in?

  10. Thrashing • If the working sets of the active processes are larger than the physical RAM then thrashing occurs Process A Working Set RAM Process B Working Set

  11. Disk Contention • When two or more processes are trying to access the HDD at the same time • Causes the HDD to jump between areas retrieving data for each process simultaneously • Becomes worse as paging increases • What can be done to help alleviate this?

  12. Paging • Windows makes an effort to improve the response time of a process by always having some free memory • Windows will page-out pages to secondary storage before there is no free RAM available • Windows decides which pages to page-out based on a least-recently-used algorithm

  13. Paging • Paging is a slow process that affect the performance of a computer • What can be done to reduce paging? • What can be done to improve the performance of paging?

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