10.1 Under what circumstances do page faults occur? Describe the actions taken by the oper-ating

system when a page fault occurs.

Ans: A page fault occurs when an access to a page that has not been brought into

main memory takes place. The operating system verifies the memory access, aborting the

table are updated and the instruction is restarted.

10.5 Assume we have a demand-paged memory. The page table is held in registers. It takes 8

milliseconds to service a page fault if an empty page is available or the replaced page is

not modified, and 20 milliseconds if the replaced page is modified. Memory access time

is 100 nanoseconds.

Assume that the page to be replaced is modified 70 percent of the time. What is the maxi-mum

acceptable page-fault rate for an effective access time of no more than 200 nanosec-onds?

10.9 Consider a demand paging system with the following time measured utilizations :

CPU utilizaton 20%

Paging disk 97.7%

Other I/O devices 5%

answer.

b. Install a bigger paging disk.

c. Increase the degree of multiprogramming.

d. Decrease the degree of multiprogramming.

e. Install more main memory.

f. Install a faster hard disk or multiple controllers with multiple hard disks.

g. Add prepaging to the page fetch algorithms.

h. Increase the page size.

The system obviously is spending most of its time paging, indicating over-allocation

of memory. If the level of multiprogramming is reduced resident processes

improve performance would be to get more physical memory or a faster paging drum.

d. Decrease the degree of multiprogramming—Yes.

remain resident and not require paging to or from the disks.

f. Install a faster hard disk, or multiple controllers with multiple hard disks—Also an

improvement, for as the disk bottleneck is removed by faster response and more

faster, so it will be more in use. This is only the case if the paging action is amenable

to prefetching (i.e., some of the access is sequential).

h. Increase the page size—Increasing the page size will result in fewer page faults if

data is being accessed sequentially. If data access is more or less random, more

paging action could ensue because fewer pages can be kept in memory and more

data is transferred per page fault. So this change is as likely to decrease utilization

as it is to increase it.

10.11 Consider the following page reference string:

1, 2, 3, 4, 2, 1, 5, 6, 2, 1, 2, 3, 7, 6, 3, 2, 1, 2, 3, 6.

How many page faults would occur for the following replacement algorithms, assuming

one, two, three, four, five, six, or seven frames? Remember all frames are initially empty,

so your first unique pages will all cost one fault each.

10.17 Consider a demand-paging system with a paging disk that has an average access and

transfer time of 20 milliseconds. Addresses are translated through a page table in main

memory, with an access time of 1 microsecond per memory access. Thus, each memory

reference through the page table takes two accesses. To improve this time, we have added

an associative memory that reduces access time to one memory reference, if the page-table

entry is in the associative memory.

Assume that 80 percent of the accesses are in the associative memory and that, of the

remaining, 10 percent (or 2 percent of the total) cause page faults. What is the effective

memory access time?

+ (0.18)(2)+ (0.02)(20002) microseconds

= 401.16 microseconds