Consider a computer system with ten physical page frames.The system is...
Introduction:
The question presents a computer system with ten physical page frames and an access sequence of virtual page numbers. We are asked to determine the difference in the number of page faults between the last-in-first-out (LIFO) page replacement policy and the optimal page replacement policy.
Explanation:
In order to understand the difference in page faults between the LIFO and optimal page replacement policies, we need to analyze how each policy handles the given access sequence.
Last-In-First-Out (LIFO) Page Replacement Policy:
In the LIFO page replacement policy, the page that has been in memory the longest is replaced when a page fault occurs. This means that the page that was most recently brought into memory will be the last to be replaced.
Optimal Page Replacement Policy:
The optimal page replacement policy, as the name suggests, replaces the page that will not be used for the longest period of time in the future. This policy requires knowledge of the future page references, which is not possible in real-time scenarios. However, for the purpose of this question, we are assuming that we have perfect knowledge of the future page references.
Analysis:
Let's analyze the given access sequence (a1;a2; :::;a20;a1;a2; :::;a20) and calculate the page faults for each policy.
- In the LIFO policy, the first ten unique virtual pages (a1 to a10) will be brought into memory without any page faults since there are ten available physical page frames.
- When a11 is accessed, a page fault will occur and the page that was most recently brought into memory (a10) will be replaced.
- Similarly, when a12 is accessed, another page fault will occur and a9 will be replaced.
- This pattern will continue until a20, resulting in ten page faults in total.
In the optimal policy, we assume that we have perfect knowledge of the future page references. Therefore, we can determine the best page to replace at each step to minimize page faults.
- When a11 is accessed, a page fault will occur and a1 will be replaced since it is the page that will not be used for the longest time in the future.
- Similarly, when a12 is accessed, a page fault will occur and a2 will be replaced.
- This pattern will continue until a20, resulting in ten page faults in total.
Conclusion:
As we can see from the analysis, both the LIFO and optimal page replacement policies result in the same number of page faults for the given access sequence. Therefore, the difference in the number of page faults between the two policies is 1.0 : 1.0, making option 'B' the correct answer.
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