ITPEC FE Morning October 2018 Question 20

Topic: Software

In the virtual memory of a paging system, which of the following is a cause that increases the number of page fault occurrences?

a)An access to a page that has not been accessed for a long time in main memoryb)An increase in access to a page that does not exist in main memoryc)An increase in access to a page that exists in main memoryd)An increase in the ratio of pages that are updated among the pages in main memory

Explanation

More accesses to pages that are not in main memory — a page fault is exactly an access to such a page.

In paged virtual memory, a page fault occurs when a program references a page that has no frame in main memory. The OS must then read the page from disk, evicting another page if memory is full. So the more often a program touches pages that are not resident, the more page faults occur; when this dominates, the system thrashes.

Answer (b)

Why not others:

•(a) — a page that is still in main memory causes no fault, however long ago it was last used; age only makes it a likely victim for LRU replacement
•(c) — accesses to resident pages are hits, so more of them lowers the page fault rate
•(d) — updated (dirty) pages must be written back to disk when replaced, which makes each replacement costlier, but they do not change how many faults occur

Key rule: A page fault is a reference to a non-resident page; the fault count depends on how often the working set does not fit in memory, not on whether resident pages are old or modified.

Software · AI-written, may contain errors

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