W8 - Virtual Memory (2)

Question 1

What are the two policies required for fetch policy?

Answer 1

1) demand paging

2) prepaging

Question 2

What actions are taken when page fault occurs?

Answer 2

1) find free frame
2) swap page into memory frame via scheduled disk operation
3) reset tables to indicate page now in memory
4) restart instruction that caused the page fault

Question 3

Describe demand paging.

Answer 3

Only brings pages into main memory when a reference is made to a location on the page.
Starts process with no pages in memory

Question 4

Describe prepaging.

Answer 4

Pages other than the one demanded by a page fault are brought in.
If pages of a process are stored contiguously in secondary memory it is more efficient to bring in a number of pages at one time
INEFFICIENT if extra pages are not referenced

Question 5

Describe placement policy.

Answer 5

Important in segmentation system.

Determines where real memory a process piece is to reside.

Question 6

Describe replacement policy.

Answer 6

When all of the frames in main memory are occupied new pages are brought in. Replacement policy determines which page currently in memory is to be replaced: page least likely to be referenced in future.

Question 7

Briefly explain the basic process to page replacement

Answer 7

Page replacement based on clock algorithm.

1) Use modify bit to reduce overhead of page transfers
2) Locate desired page on disk
3) Find free frame
4) Bring desired page into free frame
5) Restart instruction that caused trap

Question 8

What is frame locking?

Answer 8

When a frame is locked the page currently stored in that frame may not be replaced

Question 9

Name three basic replacement algorithms.

Answer 9

Optimal
Least recently used (LRU)
Clock
First-In-First-Out (FIFO)

Question 10

Explain the Least Recently Used (LRU).

Answer 10

Replaces the page that has not been referenced for the longest time
CON - difficult to implement due to high overhead requirement

Question 11

Explain FIFO as a replacement algorithm.

Answer 11

Treats page frames allocated to a process as a circular buffer.
Pages removed in RR style
IMPORTANT: Pages that have been in memory the longest is replaced

Question 12

Explain the Clock policy.

Answer 12

Each page has a ‘use bit’
If it is 0 replace it
If it is 1 give it a second chance

Question 13

Explain Enhanced Clock policy.

Answer 13

Take ordered pair (use, modify).
(0,0) neither used, neither modified
(0,1) neither used, but modified
(1,0) used, but not modified
(1,1) use and modified

Question 14

What is page buffering?

Answer 14

Keeps a small number of frames free at all time.
Required when a page fault occurs. Page is brought into memory, read into one unmodified free frame list to start process

Question 15

Resident Set Management Summary.

Answer 15

Check printed document.

Question 16

Variable-Interval Sampled Working Set (VSWS) are driven by what 3 parameters?

Answer 16

1) minimum duration of the sampling interval
2) maximum duration of sampling interval
3) number of page faults that are allowed to occur between sampling instances

Question 17

What is cleaning policy concerned about?

Answer 17

It is determining when a replaced page should be written out to secondary memory

Question 18

What is load control?

Answer 18

Determining the number of processes that will reside in main memory.
Too few processes, many occasions when all processes will be blocks and much time will be spent on swapping.
Too many processes will lead to thrashing.

Question 19

When would a process be suspended?

Answer 19

1) lowest priority process
2) largest process
3) faulting
4) last process

Question 20

What is kernel memory allocator?

Answer 20

Kernel generates and destroys small tables and buffers during course of execution - requires dynamic memory allocation