Example Questions

Question 1: Page Tables

Consider a memory management system with demand paging. You're given three processes P1, P2, P3 which have one page of private memory each. Moreover, P1 and P2 are sharing an array A which fits entirely into one memory page. Similarly, P2 and P3 are sharing an array B which fits into a memory page.

Let all the data for the processes be located into physical memory. Draw a possible memory allocation diagram, and give the page tables for the three processes.
Assume that process P1 gets swapped out of memory entirely. How are the page tables changing?
Next, assume that process P1 gets swapped back into memory. Give the page tables in this situation!
Describe an algorithm for maintaining consistent entries for shared pages in the page table. What data structures do you need to make this operations very efficient?

Question 2: Effective Access Time

Consider a memory management systme with demand paging. You know the following characteristics of the system:

TLB (Translation Lookaside Buffer) time: T_TLB = 0.1 micro second
Memory access Time: T_mem_access = 1 micro second
Time for reading from disk and updating memory: T_access_transfer = 20 milli seconds

Also, 80% of all memory accesses are served from the TLB. 10% of the remaining accesses cause a page fault.

What's the effective access time? (In other words, what's the average time it takes to serve a memory access from the above distribution?)