Operating System [3i Infotech Placement]: Sample Questions 14 - 15 of 81

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Question 14

Operating System
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Describe in Detail

Essay▾

How does kernel handle the copy on write bit of a page, when the bit is set?

Explanation

Define Copy on Write
  • In situations where the copy on write bit of a page is set and that page is shared by more than one process, the kernel allocates new page and copies the content to the new page. The other processes retain their references to the old page.
  • Then kernel decrements the reference count of the old pfdata table entry.
  • Where the copy on write bit is set and no processes are sharing the page, the kernel allows the physical page to be reused by the processes.
  • By doing so, it clears the copy on write bit and disassociates the page from its disk copy, because other process share the disk copy.
  • Then it removes the pfdata table entry from the page-queue as the new copy of the page is not on the swap device.
  • It decrements the swap-use count for the page and if count drops to 0, frees the swap space.

Question 15

Operating System
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Describe in Detail

Essay▾

What is Page-Stealer process?

Explanation

Process Virtual Address Space
  • The page eligible for swapping are found by the page-stealer and placed in a list.
  • The paging states are:
    • Not yet eligible for swapping
    • Eligible for swapping but not eligible for re assignment to other virtual address space.
  • Created by the kernel at the system initialization
  • Is invoked by kernel throughout the lifetime of the system.
  • On page fault kernel locks a region of the page so page stealer cannot steal the faulting page.

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