Lecture 17: Virtual Memory 1

Question 1

Assumption for Virtual Memory

Answer 1

degree of multiprogramming bounded by amount of disk space

for set of active processes: S(logical address spaces) <= size of disk

Question 2

Virtual Memory

Answer 2

Means:
1. High memory utilization, flexible memory allocation: allow allocated memory to be noncontiguous (chunked), out-of-order

2. Swapping and virtual memory: allow some processes to have address space dynamically mapped to physical memory

Question 3

Swapping

Answer 3

Idea:

•ready or blocked processes have no physical memory allocation. Instead, address space mapped to disk

Question 4

Swapping (+,-)

Answer 4

Achieves Higher Degree of Multiprogramming Swapping • not limited by available memory, but by available disk• modify context-switch routine to add swap-out and swap-in1.

Expensive: May need a lot of disk I/O2. Time-variable: Depends on process size

Question 5

Virtual Memory

Answer 5

Ready, blocked or running process have some (but not all)of their allotment of physical memory

Question 6

Demand Paging

Answer 6

Idea:
Demand: Bring a page into memory only when it’s needed
Paging: Units of allocation are pages (fixed size)
® less I/O needed
® less memory needed
® more users allowable

Implementation:
Straightforward extension of paging

Question 7

Page Replacement:

Answer 7

• find some page in memory but not really in use, and swap it out

Question 8

Page Fault Rate:

Answer 8

• p = 0 → no page faults

* p = 1 → every reference is a fault

Question 9

Effective Access Time

Answer 9

EAT = (1 - p) ´ memory access time
p x ( page fault overhead
\+ [swap page out]
\+ swap page in
\+ restart overhead )

Question 10

Some Typical Numbers

Answer 10

Memory Access Time: 1 microsecond
• swap page in time = swap page out time = 10,000 microseconds
• 50% of time, page being replaced has been modified (dirty) and needs to
be swapped out

Question 11

When is a page needed?

Answer 11

when it is referenced (read or written).