released: 04/01, 9:00
due: 04/06, 23:59

Answer the following questions.
Submit your answers to Canvas assignments. There is an entry for this homework.


1. Virtual memory  (5 points)

Below are five statements about virtual memory.
Write down True if you agree with the statement; otherwise, write False.

  1.a Virtual memory provides programmability (or sometimes called
      transparency) that multiple programs can use the same address say
      0x123456 without a conflict.


  1.b Each physical page can be mapped to multiple virtual pages in one process.


  1.c Each virtual page can map to multiple physical pages.


  1.d On a 32 bit machine with 8 GB RAM installed, the physical address would be
      larger than the virtual address space of one process.


  1.f TLB accelerates VM translation by assuming spatial and temporal locality.
      If the assumptions do not hold, TLB is unable to speed up VM translation.
        [see what is "locality" here: https://en.wikipedia.org/wiki/Locality_of_reference]



2. Run the mmap experiment (2 points)

Read handout week11.a page 2 (https://naizhengtan.github.io/22spring/notes/handout_w11a.pdf),
Copy the mmap code to a file "mmap.c".
Do the exercise in the handout with 1G file and answer question:

Question:
Which runs faster, option 1 or option 2? by how much on your machine?




3. Page replacement policy: CLOCK  (3 points)

Suppose you have a machine named CS5600-clover with 4 pages of memory (total 16KB) and a 1TB SSD.
A process uses 10 pages; page P0-P3 are in the memory; P4-P9 are on SSD.
OS runs CLOCK algorithm for page replacement, and a visualization is below.

      P0 (A=0)
       ^
       |
 P3    +     P1
(A=0)       (A=0)

      P2 (A=0)

For CLOCK algorithm:
- The "hand"/"pointer" will go clock-wise.
- When evicting page, check the page pointed by the "hand"
  -- if access bit is set (A=1), clear the bit (A=0) and advance one position.
  -- if A=0, evict the pointed page, load the new page, and advance one position.
  -- what's the access bit of the newly loaded page? answer: A=1
     the page is now being used.
- note: CLOCK is a family of algorithms. You might see different variants but
  the core never changes---CLOCK can be perfectly implemented on hardware.

If the memory accesses in the following are:
  P0, P2, P3, P4, P5, P0, P9

How many page swaps will happen? and for each swap, which page has been swapped
in and which page has been swapped out (namely, the victim)?