Questions and Answers of Operating System

A typical printed page of text contains 50 lines of 80 characters each. Imagine that a certain printer can print 6 pages per minute and that the time to write a character to the printer's output
Explain how an OS can facilitate installation of a new device without any need for recompiling the OS.
In which of the four I/O software layers is each of the following done. (a) Computing the track, sector, and head for a disk read. (b) Writing commands to the device registers. (c) Checking to see if
A local area network is used as follows. The user issues a system call to write data packets to the network. The operating system then copies the data to a kernel buffer. Then it copies the data to
How much cylinder skew is needed for a 7200-RPM disk with a track-to-track seek time of 1 msec? The disk has 200 sectors of 512 bytes each on each track.
RAID level 3 is able to correct single-bit errors using only one parity drive. What is the point of RAID level 2? After all, it also can only correct one error and takes more drives to do so.
A RAID can fail if two or more of its drives crash within a short time interval. Suppose that the probability of one drive crashing in a given hour is p. What is the probability of a k-drive RAID
Compare RAID level 0 through 5 with respect to read performance, write performance, space overhead, and reliability.
Why are optical storage devices inherently capable of higher data density than magnetic storage devices? Note: This problem requires some knowledge of high-school physics and how magnetic fields are
What are the advantages and disadvantages of optical disks versus magnetic disks?
If a disk controller writes the bytes it receives from the disk to memory as fast as it receives them, with no internal buffering, is interleaving conceivably useful? Discuss your answer.
If a disk has double interleaving, does it also need cylinder skew in order to avoid missing data when making a track-to-track seek? Discuss your answer.
Consider a magnetic disk consisting of 16 heads and 400 cylinders. This disk has four 100-cylinder zones with the cylinders in different zones containing 160, 200, 240. And 280 sectors, respectively.
Figure 5-3(b) shows one way of having memory-mapped I/O even in the presence of separate buses for memory and I/O devices, namely, to first try the memory bus and if that fails try the I/O bus. A
A computer manufacturer decides to redesign the partition table of a Pentium hard disk to provide more than four partitions. What are some consequences of this change?
Disk requests come in to the disk driver for cylinders 10, 22, 20, 2, 40, 6, and 38, in that order. A seek takes 6 msec per cylinder. How much seek time is needed for(a) First-come, first served.(b)
A personal computer salesman visiting a university in South-West Amsterdam remarked during his sales pitch that his company had devoted substantial effort to making their version of UNIX very fast.
In the discussion of stable storage using nonvolatile RAM, the following point was glossed over. What happens if the stable write completes but a crash occurs before the operating system can write an
In the discussion on stable storage, it was shown that the disk can be recovered to a consistent state (a write either completes or does not take place at all) if a CPU crash occurs during a write.
In the discussion on stable storage, a key assumption is that a CPU crash that corrupts a sector leads to an incorrect ECC. What problems might arise in the five crash-recovery scenarios shown in
The clock interrupt handler on a certain computer requires 2 msec (including process switching overhead) per clock tick. The clock runs at 60 Hz. What fraction of the CPU is devoted to the clock?
A system simulates multiple clocks by chaining all pending clock requests together as shown in Fig. 5-30. Suppose the current time is 5000 and there are pending clock requests for time 5008, 5012,
Explain the tradeoffs between precise and imprecise interrupts on a superscalar machine.
Many versions of UNIX use an unsigned 32-bit integer to keep track of the time as the number of seconds since the origin of time. When will these systems wrap around (year and month)? Do you expect
A bitmap terminal contains 1600 by 1200 pixels. To scroll a window, the CPU (or controller) must move all the lines of text upward by copying their bits from one part of the video RAM to another. If
After receiving a DEL (SIGINT) character, the display driver discards all output currently queued for that display. Why?
A user at a terminal issues a command to an editor to delete the word on line 5 occupying character positions 7 through and including 12. Assuming the cursor is not on line 5 when the command is
One way to place a character on a bitmapped screen is to use BitBlt from a font table. Assume that a particular font uses characters that are 16 × 24 pixels in true RGB color. (a) How much font
Assuming that it takes 2 nsec to copy a byte, how much time does it take to completely rewrite the screen of an 80 character × 25 line text mode memory-mapped screen? What about a 1024 × 768 pixel
In Fig. 5-36 there is a class to Register Class. In the corresponding X Window code, in Fig. 5-34, there is no such call or anything like it. Why not?
A DMA controller has five channels. The controller is capable of requesting a 32-bit word every 40 nsec. A response takes equally long. How fast does the bus have to be to avoid being a bottleneck?
A thin-client terminal is used to display a Web page containing an animated cartoon of size 400 pixels × 160 pixels running at 10 frames/sec. What fraction of a 100-Mbps Fast Ethernet is consumed by
Describe two advantages and two disadvantages of thin client computing?
If a CPU's maximum voltage, V, is cut to V/n, its power consumption drops to 1/n2 of its original value and its clock speed drops to 1/n of its original value. Suppose that a user is typing at 1
A notebook computer is set up to take maximum advantage of power saving features including shutting down the display and the hard disk after periods of inactivity. A user sometimes runs UNIX programs
Write a program that simulates stable storage. Use two large fixed-length files on your disk to simulate the two disks.
Suppose that a system uses DMA for data transfer from disk controller to main memory. Further assume that it takes t1 nsec on average to acquire the bus and t2 nsec to transfer one word over the bus
One mode that some DMA controllers use is to have the device controller send the word to the DMA controller, which then issues a second bus request to write to memory. How can this mode be used to
Suppose that a computer can read or write a memory word in 5 nsec. Also suppose that when an interrupt occurs, all 32 CPU registers, plus the program counter and PSW are pushed onto the stack. What
CPU architects know that operating system writers hate imprecise interrupts. One way to please the OS folks is for the CPU to stop issuing new instructions when an interrupt is signaled, but allow
Give an example of a deadlock taken from politics.
Suppose that there is a resource deadlock in a system. Give an example to show that the set of processes deadlocked can include processes that are not in the circular chain in the corresponding
In order to control traffic, a network router, A periodically sends a message to its neighbor, B, telling it to increase or decrease the number of packets that it can handle. At some point in time,
The discussion of the ostrich algorithm mentions the possibility of process-table slots or other system tables filling up. Can you suggest a way to enable a system administrator to recover from such
Consider the following state of a system with four processes, P1, P2, P3, and P4, and five types of resources, RS1, RS2, RS3, RS4, and RS5:Using the deadlock detection algorithm described in Section
Explain how the system can recover from the deadlock in previous problem using (a) Recovery through preemption. (b) Recovery through rollback. (c) Recovery through killing processes.
Suppose that in Fig. 6-6 Cij + Rij > Ej for some i. What implications does this have for the system?
In theory, resource trajectory graphs could be used to avoid deadlocks. By clever scheduling, the operating system could avoid unsafe regions. Is there a practical way of actually doing this?
Students working at individual PCs in a computer laboratory send their files to be printed by a server that spools the files on its hard disk. Under what conditions may a deadlock occur if the disk
Can a system be in a state that is neither deadlocked nor safe? If so, give an example. If not, prove that all states are either deadlocked or safe.
A system has two processes and three identical resources. Each process needs a maximum of two resources. Is deadlock possible? Explain your answer.
Consider the previous problem again, but now with p processes each needing a maximum of m resources and a total of r resources available. What condition must hold to make the system deadlock free?
The banker's algorithm is being run in a system with m resource classes and n processes. In the limit of large m and n, the number of operations that must be performed to check a state for safety is
A system has four processes and five allocatable resources. The current allocation and maximum needs are as follows:What is the smallest value of x for which this is a safe state?
One way to eliminate circular wait is to have rule saying that a process is entitled only to a single resource at any moment. Give an example to show that this restriction is unacceptable in many
Two processes, A and B, each need three records, 1, 2, and 3, in a database. If A asks for them in the order 1, 2, 3, and B asks for them in the same order, deadlock is not possible. However, if B
A distributed system using mailboxes has two IPC primitives, send and receive. The latter primitive specifies a process to receive from and blocks if no message from that process is available, even
In the preceding question, which resources are preemptable and which are nonpreemptable?
In an electronic funds transfer system, there are hundreds of identical processes that work as follows. Each process reads an input line specifying an amount of money, the account to be credited, and
One way to prevent deadlocks is to eliminate the hold-and-wait condition. In the text it was proposed that before asking for a new resource, a process must first release whatever resources it already
A computer science student assigned to work on deadlocks thinks of the following brilliant way to eliminate deadlocks. When a process requests a resource, it specifies a time limit. If the process
Main memory units are preempted in swapping and virtual memory systems. The processor is preempted in time-sharing environments. Do you think that these preemption methods were developed to handle
Explain the differences between deadlock, livelock, and starvation.
Assume two processes are issuing a seek command to reposition the mechanism to access the disk and enable a read command. Each process is interrupted before executing its read, and discovers that the
Local Area Networks utilize a media access method called CSMA/CD, in which stations sharing a bus can sense the medium and detect transmissions as well as collisions. In the Ethernet protocol,
A program contains an error in the order of cooperation and competition mechanisms, resulting in a consumer process locking a mutex (mutual exclusion semaphore) before it blocks on an empty buffer.
Cinderella and the Prince are getting divorced. To divide their property, they have agreed on the following algorithm. Every morning, each one may send a letter to the other's lawyer requesting one
The four conditions (mutual exclusion, hold and wait, no preemption and circular wait) are necessary for a resource deadlock to occur. Give an example to show that these conditions are not sufficient
City streets are vulnerable to a circular blocking condition called gridlock, in which intersections are blocked by cars that then block cars behind them that then block the cars that are trying to
Suppose four cars each approach an intersection from four different directions simultaneously. Each corner of the intersection has a stop sign. Assume that traffic regulations require that when two
Is it possible that a resource deadlock involves multiple units of one type and a single unit of another? If so, give an example.
Fig. 6-3 shows the concept of a resource graph. Do illegal graphs exist, that is, graphs that structurally violate the model we have used of resource usage? If so, give an example of one.
Consider a type 1 hypervisor that can support up to n virtual machines at the same time. PCs can have a maximum of four disk primary partitions. Can n be larger than 4? If so, where can the data be
Briefly explain the concept of process-level virtualization.
Why do type 2 hypervisors exist? After all, there is nothing they can do that type 1 hypervisors cannot do and the type 1 hypervisors are generally more efficient as well.
Is virtualization of any use to type 2 hypervisors?
Why was binary translation invented? Do you think it has much of a future? Explain your answer.
Explain how the x86's four protection rings can be used to support virtualization.
State one reason as to why a hardware-based approach using VT-enabled CPUs can perform poorly when compared to translation-based software approaches.
Give one case where a translated code can be faster than the original code, in a system using binary translation.
VMware does binary translation one basic block at a time, then it executes the block and starts translating the next one. Could it translate the entire program in advance and then execute it? If so,
What is the difference between a pure hypervisor and a pure microkernel?
Briefly explain why memory is so difficult to virtualize. well in practice? Explain your answer.
Running multiple virtual machines on a PC is known to require large amounts of memory. Why? Can you think of any ways to reduce the memory usage? Explain.
Explain the concept of shadow page tables, as used in memory virtualization.
One way to handle guest operating systems that change their page tables using ordinary (nonprivileged) instructions is to mark the page tables as read only and take a trap when they are modified. How
Why are balloon drivers used? Is this cheating?
Descibe a situation in which balloon drivers do not work.
Explain the concept of deduplication as used in memory virtualization.
Computers have had DMA for doing I/O for decades. Did this cause any problems before there were I/O MMUs?
Give one advantage of cloud computing over running your programs locally. Giv e one disadvantage as well.
Give an example of IAAS, PAAS, and SAAS.
Give a reason why a software developer might use virtualization on a desktop machine being used for development.
Why is virtual machine migration important? Under what circumstances might it be useful?
Migrating virtual machines may be easier than migrating processes, but migration can still be difficult. What problems can arise when migrating a virtual machine?
Why is migration of virtual machines from one machine to another easier than migrating processes from one machine to another?
What is the difference between live migration and the other kind (dead migration?)?
What were the three main requirements considered while designing VMware?
Why was the enormous number of peripheral devices available a problem when VMware Workstation was first introduced?
VMware ESXi has been made very small. Why? After all, servers at data centers usually have tens of gigabytes of RAM. What difference does a few tens of megabytes more or less make?

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