Lecture 3

Question 1

When do ALL algorithms perform badly?

Answer 1

When processes have insufficient physical memory.

Question 2

When do algorithms approach optimal task performance?

Answer 2

As physical memory allocated to processes approaches virtual memory size.

Question 3

Load-control strategies

Answer 3

To decide how many processes

(and pages per process) must concurrently share memory.

Question 4

What is the goal of any page frame allocation?

Answer 4

Ensure that each process scheduled to execute has a minimum number of page frames allocated, in order that enough of its pages can be loaded into memory and that the process can run.

Question 5

Global Replacement

Answer 5

The process selects replacement frame from the set of all frames,
i.e., one process can take a frame from another process.

Question 6

Local Replacement

Answer 6

Each process selects replacement frames from only its own set of allocated frames.

Question 7

Advantage of global-page replacement

Answer 7

Global-page replacement algorithms are more efficient in memory allocation because page frames are dynamically allocated among processes.

Question 8

One disadvantage of global-page replacement

Answer 8

Large computationally expensive processes

can be assigned too many memory frames.

Question 9

One Disadvantage of local page replacement

Answer 9

Processes are allocated a fixed fraction of the page frames. However, some processes may have slightly lower memory demands and other processes slightly
higher memory demands.

Question 10

Which page replacement algorithm (between local and global is generally preferred)?

Answer 10

Global page-replacement algorithms generally work better.

Question 11

Increase in memory frame allocation per process and the relationship between page faults.

Answer 11

More memory frames allocated per process, less page faults.

Few memory frames allocated per process, more page faults.

Question 12

What is the relationship between memory references (accesses) and page faults.

Answer 12

When we have more page faults there is less time for memory references, since most of the time is spent handling faults and not doing memory access.

Question 13

Free frame list

Answer 13

OS maintains a list of free memory frames in main memory.

Question 14

What happens when the number of free frames in the list falls below the threshold?

Answer 14

Page replacement occurs so that OS always has some free memory frames to assign to newly scheduled processes or processes with increasing memory demands.

Question 15

When is all of the main memory on the free frame list?

Answer 15

When the OS starts up.

Question 16

Why do we not want too many free frames to be assigned?

Answer 16

Will cause an increase in page faults if new processes are scheduled to execute.

Question 17

What happens if the number of free frames assigned to currently scheduled processes would exceed a maximum threshold?

Answer 17

OS (kernel) would suspend the reclaiming of pages until a currently scheduled process
finishes running, and the memory frames assigned to that process can be returned
to the free frame list.

Question 18

What kind of data structure is a free frame list?

Answer 18

A linked list

Question 19

Equal allocation

Answer 19

Each process receives an equal share of frames from the free frame list.

Question 20

Proportional allocation

Answer 20

Assigns a number of frames proportional to
the size (number of required pages) for each process.

Question 21

Advantage of equal allocation

Answer 21

No one process (with very large memory

demands) can monopolize memory usage.

Question 22

Advantage of proportional allocation

Answer 22

Ensures that large processes do not run too slowly and thus have their pages residing in
main memory for too long.

Question 23

priority memory allocation

Answer 23

Use of process priorities instead of process size.

All frames are assigned a priority number.

Question 24

The main issue with priority allocation

Answer 24

Priority number has to be assigned to each page and the best way to assign priority is a continuous issue.

Question 25

Page Fault Frequency (PFF)

Answer 25

The PFF metric is used with local
page replacement and works via monitoring the P F F of each process (currently
scheduled to run).

Question 26

What are the additional parameters that PFF policy introduces?

Answer 26

T and the too low and too high PFF thresholds.

Must be decided in advance and difficult to know the right values for these parameters.

Question 27

How to determine PFF parameters?

Answer 27

Measure the PFF over some
time period, where we do test runs across a range of different process load conditions
for the OS, in order to come up with some average upper and lower bound of PFF.