1.1 What are the three main purposes of an operating system?
Answer: To provide an environment for a computer user to execute
programs on computer hardware in a convenient and efficient
manner. To allocate the separate resources of the computer as
needed to solve the problem given. The allocation process should
be as fair and efficient as possible. As a control program it
serves two major functions: (1) supervision of the execution of
user programs to prevent errors and improper use of the computer,
and (2) manage- ment of the operation and control of I/O devices.
1.3 What is the main advantage of multiprogramming?
Answer: Multiprogramming makes efficient use of the CPU by
overlapping the demands for the CPU and its I/O devices from
various users. It attempts to increase CPU utilization by always
having something for the CPU to execute.
1.5 In a multiprogramming and time-sharing environment, several users
share the system si- multaneously. This situation can result in
various security problems.
a. What are two such problems?
b. Can we ensure the same degree of security in a time-shared
machine as we have in a dedicated machine? Explain your
answer.
Answer:
a. Stealing or copying one's programs or data; using system
resources (CPU, memory, disk space, peripherals) without
proper accounting.
b. Probably not, since any protection scheme devised by humans
can inevitably be bro- ken by a human, and the more complex
the scheme, the more difficult it is to feel confident of its
correct implementation.
1.6 Define the essential properties of the following types of
operating systems
a. Batch
b. Interactive
c. Time sharing
d. Real time
e. Network
f. Distributed
Answer:
a. Batch. Jobs with similar needs are batched together and run
through the computer as a group by an operator or automatic
job sequencer. Performance is increased by attempting to keep
CPU and I/O devices busy at all times through buffering,
off-line operation, spooling, and multiprogramming. Batch is
good for executing large jobs that need little interaction;
it can be submitted and picked up later.
b. Interactive. This system is composed of many short
transactions where the results of the next transaction may be
unpredictable. Response time needs to be short (seconds)
since the user submits and waits for the result.
c. Time sharing. This systems uses CPU scheduling and
multiprogramming to provide economical interactive use of a
system. The CPU switches rapidly from one user to
another. Instead of having a job defined by spooled card
images, each program reads its next control card from the
terminal, and output is normally printed immediately to the
screen.
d. Real time. Often used in a dedicated application, this system
reads information from sensors and must respond within a
fixed amount of time to ensure correct perfor- mance.
e. Network.
f. Distributed.This system distributes computation among several
physical processors. The processors do not share memory or a
clock. Instead, each processor has its own local memory. They
communicate with each other through various communication
lines, such as a high-speed bus or telephone line.
1.10 What is the main difficulty that a programmer must overcome in
writing an operating system for a real-time environment?
Answer: The main difficulty is keeping the operating system
within the fixed time con- straints of a real-time system. If the
system does not complete a task in a certain time frame, it may
cause a breakdown of the entire system it is running. Therefore
when writ- ing an operating system for a real-time system, the
writer must be sure that his scheduling schemes don't allow
response time to exceed the time constraint.
1.11 Consider the various definitions of operating system. Consider
whether the operating system should include applications such
as Web browsers and mail programs. Argue both that it should and
that it should not, and support your answer.
Answer: We will assume these applications are tightly coupled to
the kernel of the OS.
It should include these applications because if they are
important applications it may be useful to make it easy to supply
some of their functionality to other programs through system
libraries. Also tightly coupling the applications with the
kernel can increase the speed of these important applications.
It should not because the OS needs to me modular with clean
interfaces to reduce bugs and security problems.
2.3 What are the differences between a trap and an interrupt? What is
the use of each function?
Answer: An interrupt is a hardware-generated change-of-flow within
the system. An interrupt handler is summoned to deal with the
cause of the interrupt; control is then re- turned to the
interrupted context and instruction. A trap is a
software-generated interrupt. An interrupt can be used to signal
the completion of an I/O to obviate the need for device polling. A
trap can be used to call operating system routines or to catch
arithmetic errors.
2.4 For what types of operations is DMA useful? Explain your answer.
Answer: DMA is useful for transferring large quantities of data
between memory and devices. It eliminates the need for the CPU to
be involved in the transfer, allowing the transfer to complete
more quickly and the CPU to perform other tasks concurrently.
2.5 Which of the following instructions should be privileged?
a. Set value of timer.
b. Read the clock.
c. Clear memory.
d. Turn off interrupts.
e. Switch from user to monitor mode.
Answer: The following instructions should be privileged:
a. Set value of timer.
b. Clear memory.
c. Turn off interrupts.
d. Switch from user to monitor mode.
2.6 Some computer systems do not provide a privileged mode of
operation in hardware. Con- sider whether it is possible to
construct a secure operating system for these computers. Give
arguments both that it is and that it is not possible.
Answer: An operating system for a machine of this type would need
to remain in control (or monitor mode) at all times. This could be
accomplished by two methods:
a. Software interpretation of all user programs (like some
BASIC, APL, and LISP sys- tems, for example). The software
interpreter would provide, in software, what the hardware
does not provide.
b. Require meant that all programs be written in high-level
languages so that all ob- ject code is compiler-produced. The
compiler would generate (either in-line or by function calls)
the protection checks that the hardware is missing.
2.7 Some early computers protected the operating system by placing it
in a memory partition that could not be modified by either the
user job or the operating system itself. Describe two difficulties
that you think could arise with such a scheme.
Answer: The data required by the operating system (passwords,
access controls, account- ing information, and so on) would have
to be stored in or passed through unprotected memory and thus be
accessible to unauthorized users.
2.8 Protecting the operating system is crucial to ensuring that the
computer system operates correctly. Provision of this protection
is the reason behind dual-mode operation, memory protection, and
the timer. To allow maximum flexibility, however, we would also
like to place minimal constraints on the user. The following is a
list of operations that are normally protected. What is the
minimal set of instructions that must be protected?
a. Change to user mode.
b. Change to monitor mode.
c. Read from monitor memory.
d. Write into monitor memory.
e. Fetch an instruction from monitor memory.
f. Turn on timer interrupt.
g. Turn off timer interrupt.
Answer: The minimal set of instructions that must be protected are:
a. Change to monitor mode.
b. Read from monitor memory.
c. Write into monitor memory.
d. Turn off timer interrupt.
3.6 List five services provided by an operating system. Explain how
each provides conve- nience to the users. Explain also in which
cases it would be impossible for user-level pro- grams to provide
these services.
Answer:
Program execution. The operating system loads the contents (or
sections) of a file into memory and begins its execution. A
user-level program could not be trusted to properly allocate
CPU time.
I/O operations. Disks, tapes, serial lines, and other devices
must be communicated with at a very low level. The user need
only specify the device and the operation to perform on it,
while the system converts that request into device- or
controller-specific commands. User-level programs cannot be
trusted to only access devices they should have access to and
to only access them when they are otherwise unused.
File-system manipulation. There are many details in file
creation, deletion, allocation, and naming that users should
not have to perform. Blocks of disk space are used by files
and must be tracked. Deleting a file requires removing the
name file information and freeing the allocated
blocks. Protections must also be checked to assure proper file
access. User programs could neither ensure adherence to
protection methods nor be trusted to allocate only free blocks
and deallocate blocks on file deletion.
Communications. Message passing between systems requires
messages be turned into packets of information, sent to the
network controller, transmitted across a com- munications
medium, and reassembled by the destination system. Packet
ordering and data correction must take place. Again, user
programs might not coordinate ac- cess to the network device,
or they might receive packets destined for other processes.
Error detection. Error detection occurs at both the hardware
and software levels. At the hardware level, all data transfers
must be inspected to ensure that data have not been corrupted
in transit. All data on media must be checked to be sure they
have not changed since they were written to the media. At the
software level, media must be checked for data consistency;
for instance, do the number of allocated and unallocated
blocks of storage match the total number on the device. There,
errors are frequently process-independent (for instance, the
corruption of data on a disk), so there must be a global
program (the operating system) that handles all types of
errors. Also, by having errors processed by the operating
system, processes need not contain code to catch and correct
all the errors possible on a system.
3.7 What is the purpose of system calls?
Answer: System calls allow user-level processes to request
services of the operating sys- tem.
3.15 Why is the separation of mechanism and policy a desirable property?
Answer: Mechanism and policy must be separate to ensure that
systems are easy to modify. No two system installations are the
same, so each installation may want to tune the operating system
to suit its needs. With mechanism and policy separate, the policy
may be changed at will while the mechanism stays unchanged. This
arrangement provides a more flexible system.
4.1 MS-DOS provided no means of concurrent processing. Discuss three
major complications that concurrent processing adds to an operating
system.
Answer: A method of time sharing must be implemented to allow each of
several processes to have access to the system. This method involves
the preemption of processes that do not voluntarily give up the CPU
(by using a system call, for instance) and the kernel being reentrant
(so more than one process may be executing kernel code concurrently).
Processes and system resources must have protections and must be
protected from each other. Any given process must be limited in the
amount of memory it can use and the operations it can perform on
devices like disks. Care must be taken in the kernel to prevent
deadlocks between processes, so processes aren't waiting for each
other's allocated resources.
4.4 Describe the actions a kernel takes to context switch between
processes.
Answer: In general, the operating system must save the state of the
currently running process and restore the state of the process
scheduled to be run next. Saving the state of a process typically
includes the values of all the CPU registers in addition to memory
allocation. Context switches must also perform many
architecture-specific operations, including flushing data and
instruction caches.
4.5 What are the benefits and detriments of each of the following?
Consider both the systems and the programmers' levels.
a. Symmetric and asymmetric communication
b. Automatic and explicit buffering
c. Send by copy and send by reference
d. Fixed-sized and variable-sized messages
Answer:
a) Symmetric direct communication is a pain since both sides need the
name of the other process. This makes it hard to build a server.
b) Automatic makes programming easier but is a harder system to build.
c) Send by copy is better for network generalization and
synchronization issues. Send by reference is more efficient for big
data structures but harder to code because of the shared memory
implications.
d) Variable sized makes programming easier but is a harder system to
build.
4.6 The correct producer짯consumer algorithm in Section 4.4 allows only
n-1 buffers to be full at any one time. Modify the algorithm to allow
all buffers to be utilized fully.
Answer: See Chapter 7.1 page 190.
5.3 What are two differences between user-level threads and
kernel-level threads? Under what circumstances is one type better than
the other?
Answer: (1) User-level threads are unknown by the kernel, whereas the
kernel is aware of kernel threads. (2) User threads are scheduled by
the thread library and the kernel schedules kernel threads. (3) Kernel
threads need not be associated with a process whereas every user
thread belongs to a process.
5.4 Describe the actions taken by a kernel to context switch between
kernel-level threads.
Answer: Context switching between kernel threads typically
requires saving the value of the CPU registers from the thread being
switched out and restoring the CPU registers of the new thread being
scheduled.
5.5 Describe the actions taken by a thread library to context switch
between user-level threads.
Answer: Context switching between user threads is quite similar to
switching between kernel threads, although it is dependent on the
threads library and how it maps user threads to kernel threads. In
general, context switching between user threads involves taking a user
thread of its LWP and replacing it with another thread. This act
typically involves saving and restoring the state of the registers.
5.7 Assume an operating system maps user-level threads to the kernel
using the many-to-many model where the mapping is done through
LWPs. Furthermore, the system allows the developers to create
real-time threads. Is it necessary to bound a real-time thread to an
LWP? Explain.
Answer: You shouldn't have a mix of general threads and a real-time
thread all bound to a single LWP. The general threads may make a
blocking system call causing the LWP to wait and possibly miss a time
guarantee. However, this can be avoided with the many-to-many model
since there can be many LWP associated with the process.
7.4
Mutually exclusive
------------------
If we are in the critical section of process k then flag[k]=true.
Therefore if both process i and j are in their critical sections then
flag[i]=true and flag[j]=true. Without loss of generality, assume
that j is the second process to enter its critical section. The only
way it can enter is if flag[i]=false. This is a contradiction.
Progress
--------
Assume both processes are trying to get into their critical regions.
The only way a process can not get into its critical region is if it
gets stuck at one of the while loops. Without loss of generality,
assume that turn=1. In this case, if process 0 doesn't make any
progress it must get stuck at the second while loop. Also, based on
the code, flag[0]=false. Since turn=1 and flag[0]=false then process
1 can not get stuck at a while loop and it must be allowed to enter
its critical region.
Bounded Waiting
---------------
Technically, this doesn't satisfies the bounded waiting requirement.
(At least based on the definition of bounded waiting our book gives.)
Assume processes i wants into its critical region and that process j
is let in instead. When process j finishes its critical region, it
will set turn=i. This will allow process i to enter it's critical
region after it sets flag[i]=true. However until it sets
flag[i]=true, process j can go through it's critical region an
unbounded number of times. Of course, since we know process i will
eventually run and set flag[i]=true then this is probably OK.
7.9
Shared variables
int table[3]={0,0,0};
Semaphore supply=0;
Semaphore dealer=1;
void S(int i, int j)
{
while (1) {
wait(supply);
if (table[i]==1 && table[j]==1) {
table[i]=0;
table[j]=0;
smoke;
signal(dealer);
}
}
}
void D()
{
while (1) {
wait(dealer);
random(&x,&y); /*Randomly picks two values in {0,1,2} for x and y*/
table[x]=1;
table[y]=1;
signal(supply);
signal(supply);
signal(supply);
}
}
6.3 Consider the following set of processes, with the length of the
CPU-burst time given in milliseconds:
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 3
P4 1 4
P5 5 2
The processes are assumed to have arrived in the order P1, P2, P3,
P4, P5, all at time 0.
a. Draw four Gantt charts illustrating the execution of these
processes using FCFS, SJF, a nonpreemptive priority (a
smaller priority number implies a higher priority), and RR
(quantum = 1) scheduling.
b. What is the turnaround time of each process for each of the
scheduling algorithms in part a?
c. What is the waiting time of each process for each of the
scheduling algorithms in part a?
d. Which of the schedules in part a results in the minimal
average waiting time (over all processes)?
Answer:
a. The four Gantt charts are
| 1 |2|3 |4| 5 | FCS
|1|2|3|4|5|1|3|5|1|5|1|5|1|5| 1 | RR
|2|4|3 | 5 | 1 | SJF
|2| 5 | 1 | 3|4| Priority
b. Turnaround time
FCFS RR SJF Priority
P1 10 19 19 16
P2 11 2 1 1
P3 13 7 4 18
P4 14 4 2 19
P5 19 14 9 6
c. Waiting time (turnaround time minus burst time)
FCFS RR SJF Priority
P1 0 9 9 6
P2 10 1 0 0
P3 11 5 2 16
P4 13 3 1 18
P5 14 9 4 1
d. Shortest Job First
6.4 Suppose that the following processes arrive for execution at the
times indicated. Each process will run the listed amount of
time. In answering the questions, use nonpreemptive scheduling and
base all decisions on the information you have at the time the
decision must be made.
Process Arrival Time Burst Time
P1 0.0 8
P2 0.4 4
P3 1.0 1
a. What is the average turnaround time for these processes with
the FCFS scheduling algorithm?
b. What is the average turnaround time for these processes with
the SJF scheduling al- gorithm?
c. The SJF algorithm is supposed to improve performance, but
notice that we chose to run process P1 at time 0 because we
did not know that two shorter processes would arrive
soon. Compute what the average turnaround time will be if the
CPU is left idle for the first 1 unit and then SJF scheduling
is used. Remember that processes P1 and P2 are waiting during
this idle time, so their waiting time may increase. This
algorithm could be known as future-knowledge scheduling.
Answer:
a. 10.53
b. 9.53
c. 6.86
Remember that turnaround time is finishing time minus arrival time,
so you have to sub- tract the arrival times to compute the
turnaround times. FCFS is 11 if you forget to subtract arrival
time.
6.8 Many CPU scheduling algorithms are parameterized. For example, the
RR algorithm requires a parameter to indicate the time
slice. Multilevel feedback queues require parameters to define
the number of queues, the scheduling algorithms for each queue,
the criteria used to move processes between queues, and so on.
These algorithms are thus really sets of algorithms (for example,
the set of RR algorithms for all time slices, and so on). One set
of algorithms may include another (for example, the FCFS algorithm
is the RR algorithm with an infinite time quantum). What (if any)
relation holds between the following pairs of sets of algorithms?
a. Priority and SJF
b. Multilevel feedback queues and FCFS
c. Priority and FCFS
d. RR and SJF
Answer:
a. The shortest job has the highest priority.
b. The lowest level of MLFQ is often FCFS.
c. FCFS gives the highest priority to the job having been in
existence the longest.
d. If the quantum is small enough then the jobs finish in the
same order.
Note: Other answers are possible.
6.9 Suppose that a scheduling algorithm (at the level of short-term
CPU scheduling) favors those processes that have used the least
processor time in the recent past. Why will this algorithm favor
I/O-bound programs and yet not permanently starve CPU-bound
programs?
Answer: It will favor the I/O-bound programs because of the
relatively short CPU burst request by them; however, the
CPU-bound programs will not starve because the I/O-bound programs
will relinquish the CPU relatively often to do their I/O. Also
the CPU-bound programs must get at least as much CPU time as the
I/O bound programs. If they do not then the CPU-bound programs
use less processor time and the CPU bound processes become the
favored processes.
6.10 Explain the differences in the degree to which the following
scheduling algorithms discriminate in favor of short processes:
a. FCFS
b. RR
c. Multilevel feedback queues
Answer:
a. FCFS--discriminates against short jobs since any short jobs
arriving after long jobs will have a longer waiting time.
b. RR--treats all jobs equally (giving them equal bursts of CPU
time) so short jobs will be able to leave the system faster
since they will finish first.
c. Multilevel feedback queues--work similar to the RR
algorithm--they discriminate favorably toward short jobs.
Note: Other answers are possible.
7.1 What is the meaning of the term busy waiting? What other kinds of
waiting are there in an operating system? Can busy waiting be
avoided altogether? Explain your answer.
Busy waiting is waiting without giving up the CPU. A prefered
type of waiting is to wait on a non-running queue. The techniques
in the book will still give some busy waiting on the critical
sections of a semaphore. It might be possible to avoid busy
waiting by changing the critical section code of 7.2 and 7.3 so
that it blocks processes.
7.5
do {
/*Code Block A*/
while (1) {
flag[i]=want_in;
j=turn;
while (j!=i) {
if (flag[j]!=idle)
j=turn;
else
j=(j+1)%n;
}
1 flag[i]=in_cs;
j=0;
while ((j<n) && (j==i || flag[j]!=in_cs))
j++;
2 if ((j>=n) && (turn==i || flag[turn]==idle))
break;
}
3 turn=i;
critical section;
/*Code Block B*/
j=(turn+1)%n;
while (flag[j]==idle)
j=(j+1)%n;
turn=j;
flag[i]=idle;
} while(1);
Mutually exclusive
------------------
Assume two processes execute statement (3) and are currently in their
critical regions. Both of these processes must have executed
statement (1). Let Pk be the second process of these two process to
finish statement (1). This process can't be in critical section since
statement (2) must be false. Contradiction.
Progress
--------
At least one process where flag[]=want_in must get it's flag[]=in_cs.
(This is the process that is as close as possible to turn.) Consider
the first time a process has flag[]=in_cs. If none of these processes
get into their critical region then they have flag[]=want_in. Of all
these processes, only the process closest to turn can get flag[]=in_cs
a second time. Call this process k. Still, other process can have
flag[]=in_cs if they are closer to turn than process k. The same
argument for applies to these processes. If none of these processes
get into their critical region then only the one closest to turn will
get flag[]=in_cs on the next iteration through the loop. Since there
are a finite number of process, eventually only one process will have
flag[]=in_cs. This process must eventually be let into the critical
region.
Bounded Waiting
---------------
Assume process k requests its critical section by setting
flag[k]=want_in. After this occurs, the next process, call it q, that
finishes statement 3 sets turn=q and enters its critical section. No
other process can modify turn unless its just about to enter its
critical section or its leaving its critical region. When leaving the
critical section a process must execute code block 2. Code block 2
sets turn equal to the closest non_idle process. Based on the code,
if a process is waiting, turn is the number of the next process that
is allowed into its critical region. Since flag[k]=want_in,
eventually turn must be set to k. This means that process k must wait
for at most n-1 other processes to enter and leave their critical
regions.
7.12 The strict mutual exclusion within a monitor makes the bounded-buffer mon-
itor of Exercise7.11 mainly suitable for small portions.
a.Explain why this assertion is true.
b.Design a new scheme that is suitable for larger portions.
a) This question is somewhat vauge. I guess what they are getting
at is that for small buffers the buffer is often going to be
full or empty, but a big buffer is most often going to be in an
inbetween state. Therefore the mutual exclusion will really
slow down the two processes trying to access the buffer.
b) Just use the old solution where the buffer is one too big and
perhaps add some synchronization to block a process if the
buffer is full or empty to prevent spinning.
7.22 Explain the concept of transaction atomicity.
This was not covered in the assigned readings.
8.2 Is it possible to have a deadlock involving only one single
process? Explain your answer.
Answer: No. This follows directly from the hold-and-wait condition.
8.6
a) always safe
b) not always safe, run bankers to see if it's OK.
c) not always safe, run bankers to see if it's OK.
d) always safe
e) not always safe, run bankers to see if it's OK.
f) always safe
8.10 I don't understand the question.
8.13
a)
A B C D
P0 0 0 0 0
P1 0 7 5 0
P2 1 0 0 2
P3 0 0 2 0
P4 0 6 4 2
b) yes, (P0;P3;P2;P4;P1)
c) yes since it will still be in a safe state, (P0;P2;P1;P3;P4)
8.14 Consider the following resource-allocation policy. Requests and
releases for resources are allowed at any time. If a request for
resources cannot be satisfied because the resources are not
available, then we check any processes that are blocked, waiting
for resources. If they have the desired resources, then these
resources are taken away from them and are given to the
requesting process. The vector of resources for which the waiting
process is waiting is increased to include the resources that
were taken away.
For example, consider a system with three resource types and
the vector Available initialized to (4,2,2). If process P0 asks
for (2,2,1), it gets them. If P1 asks for (1,0,1), it gets
them. Then, if P0 asks for (0,0,1), it is blocked (resource not
available). If P2 now asks for (2,0,0), it gets the available one
(1,0,0) and one that was allocated to P0 (since P0 is blocked).
P0's Allocation vector goes down to (1,2,1), and its Need vector
goes up to (1,0,1).
a. Can deadlock occur? If so, give an example. If not, which
necessary condition cannot occur?
b. Can indefinite blocking occur?
Answer:
a. Deadlock cannot occur because preemption exists.
b. Yes. A process may never acquire all the resources it needs
if they are continuously preempted by a series of requests
such as those of process C.
Additional problem
Shared Memory
-------------
semaphore SA=1;
semaphore SB=0;
process A()
{
wait(SA);
critical section
signal(SB);
}
process B()
{
wait(SB);
critical section
signal(SA);
}
9.1 Name two differences between logical and physical addresses.
Answer: Logical address is an address seen by the CPU while a
physical address is seen by the memory. A physical address is
limited to the amount of installed memory while a logical address
is limited by the address size of the processor.
9.2 Explain the difference between internal and external fragmentation.
Answer: Internal Fragmentation is the area in a region or a page
that is not used by the job occupying that region or page. This
space is unavailable for use by the system until that job is
finished and the page or region is released. External
fragmentation is the area or region that is not used because it is free.
9.5 Given memory partitions of 100K, 500K, 200K, 300K, and 600K (in
order), how would each of the First-fit, Best-fit, and Worst-fit
algorithms place processes of 212K, 417K, 112K, and 426K (in
order)? Which algorithm makes the most efficient use of memory?
Answer:
a. First-fit:
212K is put in 500K partition
417K is put in 600K partition
112K is put in 288K partition (new partition 288K = 500K - 212K)
426K must wait
b. Best-fit:
212K is put in 300K partition
417K is put in 500K partition
112K is put in 200K partition
426K is put in 600K partition
c. Worst-fit:
212K is put in 600K partition
417K is put in 500K partition
112K is put in 388K partition
426K must wait
In this example, Best-fit turns out to be the best.
9.7 Why are page sizes always powers of 2?
Answer: Recall that paging is implemented by breaking up an
address into a page and offset number. It is most efficient to
break the address into X page bits and Y offset bits, rather than
perform arithmetic on the address to calculate the page number and
offset. Because each bit position represents a power of 2,
splitting an address between bits results in a page size that is a
power of 2.
9.8 Consider a logical address space of eight pages of 1024 words each, mapped onto a physi-
cal memory of 32 frames.
a. How many bits are there in the logical address?
b. How many bits are there in the physical address?
Answer:
a. Logical address: 13 bits
b. Physical address: 15 bits
9.10 Consider a paging system with the page table stored in memory.
a. If a memory reference takes 200 nanoseconds, how long does a
paged memory refer- ence take?
b. If we add associative registers, and 75 percent of all
page-table references are found in the associative
registers, what is the effective memory reference time?
(Assume that finding a page-table entry in the associative
registers takes zero time, if the entry is there.)
Answer:
a. 400 nanoseconds; 200 nanoseconds to access the page table
and 200 nanoseconds to access the word in memory.
b. Effective access time = 0.75 (200 nanoseconds) + 0.25 (400
nanoseconds) = 250 nanoseconds.
10.3 A certain computer provides its users with a virtual-memory space
of 2^32 bytes. The computer has 2^18 bytes of physical
memory. The virtual memory is implemented by paging, and the page
size is 4096 bytes. A user process generates the virtual address
11123456. Explain how the system establishes the corresponding
physical location. Distinguish between software and hardware
operations.
Answer: The virtual address in binary form is
0001 0001 0001 0010 0011 0100 0101 0110
Since the page size is 2^12, the page table size is 2^20. Therefore
the low-order 12 bits "0100 0101 0110" are used as the
displacement into the page, while the remaining 20 bits "0001
0001 0001 0010 0011" are used as the displacement in the page
table.
10.4 Which of the following programming techniques and structures are "good" for a demand-
paged environment ? Which are "not good"? Explain your answers.
a. Stack
b. Hashed symbol table
c. Sequential search
d. Binary search
e. Pure code
f. Vector operations
g. Indirection
Answer:
a. Stack--good.
b. Hashed symbol table--not good.
c. Sequential search--good.
d. Binary search--not good.
e. Pure code--good.
f. Vector operations--good.
g. Indirection--not good.
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?
Answer: All work is done in micro seconds.
.2 = .1 + P(.3(8000) + .7(20000)
P = 6.1*10-6
10.8 An operating system supports a paged virtual memory, using a
central processor with a cycle time of 1 microsecond. It costs an
additional 1 microsecond to access a page other than the current
one. Pages have 1000 words, and the paging device is a drum that
rotates at 3000 revolutions per minute and transfers 1 million
words per second. The following statistical measurements were
obtained from the system:
1 percent of all instructions executed accessed a page other
than the current page. Of the instructions that accessed
another page, 80 percent accessed a page already in memory.
When a new page was required, the replaced page was modified 50
percent of the time. Calculate the effective instruction time
on this system, assuming that the system is run- ning one
process only, and that the processor is idle during drum
transfers.
Answer: All work is done in micro seconds
Access time = .99(1) + .01(.8(2) + .2(.5(11000) + .5(22000)))
= 34
10.9 Consider a demand-paging system with the following time-measured
utilizations:
CPU utilization 20%
Paging disk 97.7%
Other I/O devices 5%
Which (if any) of the following will (probably) improve CPU
utilization? Explain your answer.
a. Install a faster CPU.
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.
Answer: The system obviously is spending most of its time paging,
indicating over allocation of memory. If the level of multiprogramming
is reduced resident processes would page fault less frequently and the
CPU utilization would improve. Another way to improve performance
would be to get more physical memory or a faster paging drum.
a. Get a faster CPU--No.
b. Get a bigger paging drum--No.
c. Increase the degree of multiprogramming--No.
d. Decrease the degree of multiprogramming--Yes.
e. Install more main memory--Likely to improve CPU utilization as
more pages can 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 throughput to the disks, the
CPU will get more data more quickly.
g. Add prepaging to the page fetch algorithms--Again, the CPU will
get more data 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.
LRU replacement
FIFO replacement
Optimal replacement
Answer:
Number of frames LRU FIFO Optimal
1 20 20 20
2 18 18 15
3 15 16 11
4 10 14 8
5 8 10 7
6 7 10 7
7 7 7 7
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?
Answer: All work is done in micro seconds
Access time = .8(1) + .2(.9(2) + .1(20002))
= 401.2
10.18 Consider a demand-paged computer system where the degree of
multiprogramming is currently fixed at four. The system was
recently measured to determine utilization of CPU and the paging
disk. The results are one of the following alternatives. For
each case, what is happening? Can the degree of multiprogramming
be increased to increase the CPU utilization? Is the paging
helping?
a. CPU utilization 13 percent; disk utilization 97 percent
b. CPU utilization 87 percent; disk utilization 3 percent
c. CPU utilization 13 percent; disk utilization 3 percent
Answer:
a. Thrashing is occurring.
b. CPU utilization is sufficiently high to leave things alone
c. Increase the degree of multiprogramming.
9.16 Consider the following segment table:
Segment Base Length
0 219 600
1 2300 14
2 90 100
3 1327 580
4 1952 96
What are the physical addresses for the following logical addresses?
a. 0,430 b. 1,10 c. 2,500 d. 3,400 e. 4,112
Answer:
a. 219 + 430 = 649 b. 2300 + 10 = 2310 c. illegal reference, trap to
operating system d. 1327 + 400 = 1727 e. illegal reference, trap
to operating system
13.2 Consider the following I/O scenarios on a single-user PC.
a. A mouse used with a graphical user interface
b. A tape drive on a multitasking operating system (assume no device
preallocation is available)
c. A disk drive containing user files
d. A graphics card with direct bus connection, accessible through
memory-mapped I/O
For each of these I/O scenarios, would you design the operating system
to use buffering, spooling, caching, or a combination? Would you use
polled I/O, or interrupt-driven I/O? Give reasons for your choices.
Answer:
a. A mouse used with a graphical user interface Buffering may be
needed to record mouse movement during times when higher- priority
operations are taking place. Spooling and caching are
inappropriate. Inter- rupt driven I/O is most appropriate.
b. A tape drive on a multitasking operating system (assume no device
preallocation is available) Buffering may be needed to manage
throughput difference between the tape drive and the source or
destination of the I/O, Caching can be used to hold copies of data
that resides on the tape, for faster access. Spooling could be used to
stage data to the device when multiple users desire to read from or
write to it. Interrupt driven I/O is likely to allow the best
performance.
c. A disk drive containing user files. Buffering can be used to hold
data while in transit from user space to the disk, and visa
versa. Caching can be used to hold disk-resident data for improved
perfor- mance. Spooling is not necessary because disks are
shared-access devices. Interrupt- driven I/O is best for devices such
as disks that transfer data at slow rates.
d. A graphics card with direct bus connection, accessible through
memory-mapped I/O Buffering may be needed to control multiple access
and for performance (double- buffering can be used to hold the next
screen image while displaying the current one). Caching and spooling
are not necessary due to the fast and shared-access natures of the
device. Polling and interrupts are only useful for input and for I/O
completion detection, neither of which is needed for a memory-mapped
device.
13.4 Describe three circumstances under which blocking I/O should be
used. Describe three circumstances under which nonblocking I/O should
be used. Why not just implement nonblocking I/O and have processes
busy-wait until their device is ready?
Answer:
Generally, blocking I/O is appropriate when the process will only be
waiting for one spe- cific event. Examples include a disk, tape, or
keyboard read by an application program. Non-blocking I/O is useful
when I/O may come from more than one source and the order of the I/O
arrival is not predetermined. Examples include network daemons
listening to more than one network socket, window managers that accept
mouse movement as well as keyboard input, and I/O-management programs,
such as a copy command that copies data between I/O devices. In the
last case, the program could optimize its performance by buffering the
input and output and using non-blocking I/O to keep both devices fully
occupied.
Non-blocking I/O is more complicated for programmers, because of the
asynchonous rendezvous that is needed when an I/O occurs. Also, busy
waiting is less efficient than interrupt-driven I/O so the overall
system performance would decrease.
13.5 Why might a system use interrupt-driven I/O to manage a single
serial port, but polling I/O to manage a front-end processor, such as
a terminal concentrator?
Answer: Polling can be more efficient than interrupt-driven I/O. This
is the case when the I/O is frequent and of short duration. Even
though a single serial port will perform I/O relatively infrequently
and should thus use interrupts, a collection of serial ports such as
those in a terminal concentrator can produce a lot of short I/O
operations, and interrupting for each one could create a heavy load on
the system. A well-timed polling loop could alleviate that load
without wasting many resources through looping with no I/O needed.
13.6 Polling for an I/O completion can waste a large number of CPU
cycles if the processor iterates a busy-waiting loop many times before
the I/O completes. But if the I/O device is ready for service, polling
can be much more efficient than is catching and dispatching an
interrupt. Describe a hybrid strategy that combines polling, sleeping,
and interrupts for I/O device service. For each of these three
strategies (pure polling, pure interrupts, hybrid), describe a
computing environment in which that strategy is more efficient than is
either of the others.
Answer: For a hybrid put a limit on the number of loops spent in a
busy loop. Let C be the amount of time it takes to catch and dispatch
an interupt for handing I/O. Set the limit so that the time busy
waiting is C. If the I/O is not ready after the loops are done put
the process to sleep and wait for an interupt to signal that the I/O
is done.
Let 3/4 of the I/Os take C/2 time and 1/4 of the I/0s take 3C/2 time.
* average processor time wasted for polling = 3C/8 + 3C/8 = 6C/8
average processor time wasted for interupts = C
average processor time wasted for hybrid = 3C/8 + C/2 = 7C/8
Let all of the I/Os take 8C time.
average processor time wasted for polling = 8C
* average processor time wasted for interupts = C
average processor time wasted for hybrid = 2C
Let 3/4 of the I/Os take C/2 time and 1/4 of the I/0s take 8C time.
average processor time wasted for polling = 3C/8 + 2C = 19C/8
average processor time wasted for interupts = C
* average processor time wasted for hybrid = 3C/8 + C/2 = 7C/8
Virtual Memory (25 points)
Consider the following paged segmentation scheme
Segment Table
Segment Number Page Table
0 2
1 0
2 1
Page Table 1
Virtual Page Frame# Valid Read Only?
0 3 N N
1 22 N N
2 73 Y N
3 74 Y N
4 85 Y N
5 29 Y N
6 63 Y N
7 93 Y N
8 83 Y N
9 15 Y N
10 27 Y N
11 34 Y N
Page Table 0
Virtual Page Frame# Valid Read Only?
0 10 Y N
1 17 N N
2 89 Y N
3 90 Y N
4 29 Y N
5 47 N N
6 55 Y N
7 32 Y N
8 36 Y N
9 9 Y N
Page Table 2
Virtual Page Frame# Valid Read Only?
0 33 Y Y
1 46 Y Y
2 54 N Y
3 6 Y Y
4 99 N Y
5 67 Y Y
6 21 Y Y
"Valid" is set to Y if the page is in memory
"Read Only" is set to Y if the page is not writable
Given the following parameters:
A page size of 1000 words using base 10 integer addresses (not
realistic, but it makes the math easier). Thus, the first page of
virtual memory runs from virtual address 0 through virtual address
999. (All accesses to memory are for 4-byte words).
The maximum number of page table entries is 100 (0 to 99)
Each process is separated into at most 3 segments (1 segment each for
code, heap allocated data, and the stack).
If the page is invalid (Valid = N), assume the page fault causes the
OS to load the faulting page into the given frame number; for example,
an access to virtual page 2 in page table 2 causes a page fault where
the OS loads virtual page 2 into frame 54.
(a) (10 points) How many integer digits do we need for the segment
number, page number, and offset?
(b) What physical addresses do the following virtual addresses access?
(Assume any missing digits are leading zeroes). Indicate if the
access generates an error (invalid segment, invalid page, or
protection violation) or a page fault.
i. (3 points) Read from virtual address 211333
ii. (3 points) Write to virtual address 5345
iii.(3 points) Read from virtual address 1810627
iv. (3 points) Read from virtual address 104806
v. (3 points) Read from virtual address 200097
Answers:
a) segment 1 digit, page 2 digits, offset 3 digits.
b) i. 34333
ii. 67345 error readonly
iii. error segment out of range
iv. 29806
v. 3097 page fault
11.1 Consider a file system where a file can be deleted and its disk
space reclaimed while links to that file still exist. What
problems may occur if a new file is created in the same storage
area or with the same absolute path name? How can these problems
be avoided?
Answer: Let F1 be the old file and F2 be the new file. A user
wishing to access F1 through an existing link will actually
access F2. Note that the access protection for file F1 is used
rather than the one associated with F2. This problem can be
avoided by insuring that all links to a deleted file are deleted
also. This can be accomplished in several ways:
a. maintain a list of all links to a file, removing each of
them when the file is deleted
b. retain the links, removing them when an attempt is made to
access a deleted file
c. maintain a file reference list (or counter), deleting the
file only after all links or refer- ences to that file have
been deleted.
11.3 Why do some systems keep track of the type of a file, while
others leave it to the user or simply do not implement multiple
file types? Which system is "better?"
Answer: Some systems allow different file operations based on the
type of the file (for instance, an ascii file can be read as a
stream while a database file can be read via an index to a
block). Other systems leave such interpretation of a file's data
to the process and provide no help in accessing the data. The
method which is "better" depends on the needs of the processes on
the system, and the demands the users place on the operating
system. If a system runs mostly database applications, it may be
more efficient for the operating system to implement a
database-type file and provide operations, rather than making
each program implement the same thing (possibly in different
ways). For general purpose systems it may be better to only
implement basic file types to keep the operating system size
smaller and allow maximum freedom to the processes on the system.
11.5 What are the advantages and disadvantages of recording the name
of the creating pro- gram with the file's attributes (as is done
in the Macintosh Operating System)?
Answer: By recording the name of the creating program, the
operating system is able to implement features (such as automatic
program invocation when the file is accessed) based on this
information. It does add overhead in the operating system and
require space in the file descriptor, however.
11.8 Some systems automatically open a file when it is referenced for
the first time, and close the file when the job
terminates. Discuss the advantages and disadvantages of this
scheme as compared to the more traditional one, where the user
has to open and close the file explicitly.
Answer: Automatic opening and closing of files relieves the user
from the invocation of these functions, and thus makes it more
convenient to the user; however, it requires more overhead than
the case where explicit opening and closing is required.
11.12 Consider a system that supports 5000 users. Suppose that you
want to allow 4990 of these users to be able to access one file.
a. How would you specify this protection scheme in UNIX?
b. Could you suggest another protection scheme that can be
used more effectively for this purpose than the scheme
provided by UNIX?
Answer:
a. There are two methods for achieving this:
i. Create an access control list with the names of all 4990
users.
ii. Put these 4990 users in one group and set the group
access accordingly. This scheme cannot always be
implemented since user groups are restricted by the
system.
b. The universe access information applies to all users unless
their name appears in the access-control list with
different access permission. With this scheme you simply
put the names of the remaining ten users in the access
control list but with no access privileges allowed.
12.1 Consider a file currently consisting of 100 blocks. Assume that
the file control block (and the index block, in the case of indexed
allocation) is already in memory. Calculate how many disk I/O
operations are required for contiguous, linked, and indexed
(single-level) allocation strategies, if, for one block, the following
conditions hold. In the contiguous- allocation case, assume that there
is no room to grow in the beginning, but there is room to grow in the
end. Assume that the block information to be added is stored in
memory.
a. The block is added at the beginning.
b. The block is added in the middle.
c. The block is added at the end.
d. The block is removed from the beginning.
e. The block is removed from the middle.
f. The block is removed from the end.
Answer:
Contiguous Linked Indexed
a. 201 1 1
b. 101 52 1
c. 1 3 1
d. 198 1 0
e. 98 52 0
f. 0 100 0
12.2 Consider a system where free space is kept in a free-space list.
a. Suppose that the pointer to the free-space list is lost. Can the
system reconstruct the free-space list? Explain your answer.
b. Suggest a scheme to ensure that the pointer is never lost as a
result of memory failure.
Answer:
a. In order to reconstruct the free list, it would be necessary to
perform "garbage collection." This would entail searching the entire
directory structure to determine which pages are already allocated to
jobs. Those remaining unallocated pages could be relinked as the
free-space list.
b. The free-space list pointer could be stored on the disk, perhaps in
several places.
12.6 Consider a file system on a disk that has both logical and
physical block sizes of 512 bytes. Assume that the information about
each file is already in memory. For each of the three allocation
strategies (contiguous, linked, and indexed), answer these questions:
a. How is the logical-to-physical address mapping accomplished in this
system? (For the indexed allocation, assume that a file is always less
than 512 blocks long.)
b. If we are currently at logical block 10 (the last block accessed
was block 10) and want to access logical block 4, how many physical
blocks must be read from the disk?
Answer: Let Z be the starting file address (block number).
a. Contiguous. Divide the logical address by 512 with X and Y the
resulting quotient and remainder respectively.
i. Add X to Z to obtain the physical block number. Y is the
displacement into that block.
ii. 1
b. Linked. Divide the logical physical address by 511 with X
and Y the resulting quotient and remainder respectively.
i. Chase down the linked list (getting X + 1 blocks). Y + 1 is the
displacement into the last physical block.
ii. 4
c. Indexed. Divide the logical address by 512 with X and Y the
resulting quotient and remainder respectively.
i. Get the index block into memory. Physical block address is
contained in the index block at location X. Y is the
displacement into the desired physical block.
ii. 2
12.7 One problem with contiguous allocation is that the user must
preallocate enough space for each file. If the file grows to be
larger than the space allocated for it, special actions must be
taken. One solution to this problem is to define a file structure
consisting of an initial contiguous area (of a specified
size). If this area is filled, the operating system automatically
defines an overflow area that is linked to the initial contiguous
area. If the overflow area is filled, another overflow area is
allocated. Compare this implementation of a file with the
standard contiguous and linked implementations.
Answer: This method requires more overhead then the standard
contiguous allocation. It requires less overhead than the
standard linked allocation.
12.14 Consider the following backup scheme:
Day 1. Copy to a backup medium all files from the disk.
Day 2. Copy to another medium all files changed since day 1.
Day 3. Copy to another medium all files changed since day 1.
This contrasts to the schedule given in Section 11.6.2 by having
all subsequent backups copy all files modified since the first
full backup. What are the benefits of this system over the one in
Section 11.6.2? What are the drawbacks? Are restore operations
made easier or more difficult? Explain your answer.
Answer: Restores are easier because you can go to the last backup
tape, rather than the full tape. No intermediate tapes need be
read. More tape is used as more files change.
14.1 None of the disk-scheduling disciplines, except FCFS, is truly
fair (starvation may occur).
a. Explain why this assertion is true.
b. Describe a way to modify algorithms such as SCAN to ensure fairness.
c. Explain why fairness is an important goal in a time-sharing system.
d. Give three or more examples of circumstances in which it
is important that the op- erating system be unfair in
serving I/O requests.
Answer:
a. New requests for the track over which the head currently resides
can theoretically arrive as quickly as these requests are being
serviced.
b. All requests older than some predetermined age could be "forced"
to the top of the queue, and an associated bit for each could be
set to indicate that no new request could be moved ahead of
these requests. For SSTF, the rest of the queue would have to be
reorganized with respect to the last of these "old" requests.
c. To prevent unusually long response times.
d. Paging and swapping should take priority over user requests. It
may be desirable for other kernel-initiated I/O, such as the
writing of file system metadata, to take precedence over user
I/O. If the kernel supports real-time process priorities, the
I/O requests of those processes should be favored.
14.2 Suppose that a disk drive has 5000 cylinders, numbered 0 to
4999. The drive is currently serving a request at cylinder 143,
and the previous request was at cylinder 125. The queue of
pending requests, in FIFO order, is
86, 1470, 913, 1774, 948, 1509, 1022, 1750, 130
Starting from the current head position, what is the total
distance (in cylinders) that the disk arm moves to satisfy all the
pending requests, for each of the following disk- scheduling
algorithms?
a. FCFS
b. SSTF
c. SCAN
d. LOOK
e. C-SCAN
f. C-LOOK
Answer:
a. The FCFS schedule is 143, 86, 1470, 913, 1774, 948, 1509,
1022, 1750, 130. The total seek distance is 7081.
b. The SSTF schedule is 143, 130, 86, 913, 948, 1022, 1470,
1509, 1750, 1774. The total seek distance is 1745.
c. The SCAN schedule is 143, 913, 948, 1022, 1470, 1509, 1750,
1774, 4999, 130, 86. The total seek distance is 9769.
d. The LOOK schedule is 143, 913, 948, 1022, 1470, 1509, 1750,
1774, 130, 86. The total seek distance is 3319.
e. The C-SCAN schedule is 143, 913, 948, 1022, 1470, 1509,
1750, 1774, 4999, 0, 86, 130. The total seek distance is
9985.
f. The C-LOOK schedule is 143, 913, 948, 1022, 1470, 1509,
1750, 1774, 86, 130. The total seek distance is 3363.
14.10 Requests are not usually uniformly distributed. For example, a
cylinder containing the file system FAT or inodes can be
expected to be accessed more frequently than a cylinder that
only contains files. Suppose you know that 50 percent of the
requests are for a small, fixed number of cylinders.
a. Would any of the scheduling algorithms discussed in this
chapter be particularly good for this case? Explain your
answer.
b. Propose a disk-scheduling algorithm that gives even better
performance by taking advantage of this "hot spot" on the
disk.
c. File systems typically find data blocks via an indirection
table, such as a FAT in DOS or inodes in UNIX. Describe
one or more ways to take advantage of this indirection to
improve the disk performance.
Answer:
a. SSTF would take greatest advantage of the situation. FCFS
could cause unnecessary head movement if references to the
"high-demand" cylinders were interspersed with references
to cylinders far away.
b. Here are some ideas. Place the hot data near the middle of
the disk. Modify SSTF to prevent starvation. Add the
policy that if the disk becomes idle for more than, say,
50 ms, the operating system generates an anticipatory seek
to the hot region, since the next request is more likely
to be there.
c. Cache the metadata in primary memory, and locate a file's
data and metadata in close physical proximity on the
disk. (UNIX accomplishes the latter goal by allocat- ing
data and metadata in regions called cylinder groups.)
14.11 Why is rotational latency usually not considered in disk
scheduling? How would you modify SSTF, SCAN, and C-SCAN to
include latency optimization?
Answer: Most disks do not export their rotational position
information to the host. Even if they did, the time for this
information to reach the scheduler would be subject to
imprecision and the time consumed by the scheduler is variable,
so the rotational position information would become
incorrect. Further, the disk requests are usually given in terms
of logical block numbers, and the mapping between logical blocks
and physical locations is very complex.
14.14 What are the tradeoffs involved in rereading code pages from the
file system versus using swap space to store them?
Answer: If code pages are stored in swap space, they can be
transferred more quickly to main memory (because swap space
allocation is tuned for faster performance than general file
system allocation). Using swap space can require startup time if
the pages are copied there at process invocation rather than
just being paged out to swap space on demand. Also, more swap
space must be allocated if it is used for both code and data
pages.