In the optimal page replacement algorithm that we would like to have (the ideally optimal algorithm in theory) , when a page needs to be replaced because the memory is full, which of the following pages is chosen to be swapped out from memory to disk?

Question

BlackTom AI · Accepted Answer

Belady’s optimal (OPT) page replacement policy selects the page whose next use is farthest in the future — equivalently, the page that will be accessed least (or not at all) in the upcoming reference string, assuming perfect knowledge of future references.

Relevant concepts:
- Page replacement: when physical memory frames are full and a new page must be loaded, one resident page must be evicted.
- Optimal (Belady’s) algorithm: with full knowledge of future references, the best choice is the page whose next reference occurs the farthest ahead (or never). This minimizes future page faults.

Step-by-step reasoning:
1. Given the current set of pages in memory and the future reference sequence, look forward from the current point.
2. For each resident page, determine when (if at all) it will next be referenced. If a page is never referenced again, its next-use time is effectively infinite.
3. Compare these next-use times (or counts of future uses). The page with the longest time until next use — i.e., the one that will be accessed least in the near future — should be evicted.
4. Evict that page and load the requested page; this choice ensures the fewest possible page faults over the remainder of the sequence.

Illustrative example: frames hold {A, B, C}. Future references from now: A, D, B, A, ... Next uses: A at position 1, B at position 3, C never. C’s next use is infinite, so evict C.

Why the correct option is correct:
- “Least frequently accessed page in the future (assuming we know the future)” captures the OPT idea: evict the page with the minimal upcoming usage (including none). That yields the minimal number of future faults.

Why other options are incorrect:
- “Oldest page that was swapped in” describes FIFO, not optimal; FIFO can be suboptimal.
- “Newest page that was swapped in” (LIFO-like) is not optimal and generally performs poorly.
- “Most frequently accessed page in the future” is wrong because evicting a highly used future page would cause many faults.

Conclusion: The correct choice is “least frequently accessed page in the future (assuming we know the future),” which matches the provided answer.

In the optimal page replacement algorithm that we would like to have (the ideally optimal algorithm in theory) , when a page needs to be replaced because the memory is full, which of the following pages is chosen to be swapped out from memory to disk?单项选择题

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