GATE CS 2020 Set 1 — Question 63
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Operating System → Memory Management → Demand Paging
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Question
Consider a paging system that uses 1-level page table residing in main memory and a TLB for address translation. Each main memory access takes 100 ns and TLB lookup takes 20 ns. Each page transfer to/from the disk takes 5000 ns. Assume that the TLB hit ratio is 95%, page fault rate is 10%. Assume that for 20% of the total page faults, a dirty page has to be written back to disk before the required page is read in from disk. TLB update time is negligible. The average memory access time in ns (round off to 1 decimal places) is _______.
Correct answer
154.5 to 155.5
Solution
Let the parameters be:
When there is a TLB hit, we perform a TLB lookup and then access the main memory for data.
nsStep 2: Calculate Effective Access Time for TLB Miss
When there is a TLB miss (), we must access the page table in main memory ().
After accessing the page table, there are two possibilities:
Substituting the values:
nsRounding to 1 decimal place: 155.0
- TLB lookup time, ns
- Main memory access time, ns
- Disk transfer time, ns
- TLB Hit ratio,
- Page Fault rate (given TLB miss),
- Dirty page probability,
When there is a TLB hit, we perform a TLB lookup and then access the main memory for data.
nsStep 2: Calculate Effective Access Time for TLB Miss
When there is a TLB miss (), we must access the page table in main memory ().
After accessing the page table, there are two possibilities:
1.Page Hit (No Page Fault): The page is in memory. We then access the data in memory.
- Probability:
- Time: ns
- Probability:
- We must service the page fault. The time depends on whether the victim page is dirty.
- Dirty Page Service Time: Write back + Read new = ns
- Clean Page Service Time: Read new = ns
- Average Page Fault Service Time () = ns
- Total Time for Fault Case: ns
Substituting the values:
nsRounding to 1 decimal place: 155.0
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