Awesome q2a theme
0 votes
Assume a system with 2-levels of paging with the main memory access time of 100ns, and the cache lookup time of 50ns. Assuming a cache hit rate of 90% and no parallel memory access , what is the average time to read a location from main memory? (in nanoseconds)

the answer mentioned is 180ns and sugested explanation says 3 memory accesses would be required however i got my answer as 60nsec.

can someone please help me with this.
ago in Operating System by (20 points) | 33 views
No,I don't think by default we assume page tables in cache...they should mention it explicitly.
I feel that the answer revolves around hierarchical access of memory also no overlapping of operations.

since the question says cannot be accessed parallely it might even mean that the data cannot even be read parallel that is each required word has to be accessed individually, therefore, they might have multiplied it by 3.

which gives us 60*3 as the answer.this is all that i can conclude from all the responses.
what is wrong if i compute it this way?

0.9*(50+100) + 0.1*350
Firstly - that is a whole lot of assumptions you are making simply to justify the given answer. Remember - solutions come from concepts. Not concepts from solutions.

Secondly, assuming page tables are cached - 0.9*(50+100) + 0.1*350 - the problem with this is that you are assuming both levels of page table will be available in the cache at all times, or none will be available. Whereas in reality, it is entirely possible for one level of page table to be in cache, and the other level being a cache miss. This solution does not account for that.

Anyway - if you understand the concept, move on to the next one. Don't try to justify the given answer by any means necessary and risk picking up wrong concepts :)
thankyou for the response,it really did helped :)

Please log in or register to answer this question.

Quick search syntax
tags tag:apple
author user:martin
title title:apple
content content:apple
exclude -tag:apple
force match +apple
views views:100
score score:10
answers answers:2
is accepted isaccepted:true
is closed isclosed:true
Welcome to GATE CSE Doubts, where you can ask questions and receive answers from other members of the community.
Top Users Jan 2020
  1. shashin

    1163 Points

  2. Vimal Patel

    307 Points

  3. Deepakk Poonia (Dee)

    305 Points

  4. Debapaul

    237 Points

  5. Satbir

    192 Points

  6. SuvasishDutta

    137 Points

  7. Pratyush Priyam Kuan

    118 Points

  8. tp21

    108 Points

  9. DukeThunders

    96 Points

  10. pranay562

    95 Points

Monthly Top User and those within 60% of his/her points will get a share of monthly revenue of GO subject to a minimum payout of Rs. 500. Current monthly budget for Top Users is Rs. 75.
2,989 questions
1,509 answers
89,814 users