ICS 462 Midterm - Prof Study Guide

Consider the following page-replacement algorithms. Rank these algorithms on a five-point scale from "bad" to "perfect" according to their page-fault rate. Separate those algorithms that suffer from Belady's anomaly from those that do not. a.LRU replacement b.FIFO replacement c.Optimal replacement d.Second-chance replacement

1 Optimal, 2 LRU, 3 Second-chance, 4 FIFO Optimal and LRU do not suffer from Belady's anomaly, others do

What is a bootstrap program and where is it stored?

A bootstrap program is the initial program that the computer runs when it is powered up or rebooted. It initializes all aspects of the system, from CPU registers to device controllers to memory contents. Typically, it is stored in read-only memory (ROM) or electrically erasable programmable read-only memory (EEPROM), known by the general term firmware, within the computer hardware.

If a cache can be made as large as the device for which it is caching (for instance, a cache as large as a disk), why not make it that large and eliminate the device?

A component may be eliminated by an equal-sized cache, but only if: (a) the cache and the component have equivalent state-saving capacity (that is, if the component retains its data when electricity is removed, the cache must retain data as well), and(b) the cache is affordable, because faster storage tends to be more expensive.

Describe why multi-core processing is more efficient than placing each processor on its own chip.

A large reason why it is more efficient is that communication between processors on the same chip is faster than processors on separate chips.

Name two differences between logical and physical addresses.

A logical address does not refer to an actual existing address; rather, it refers to an abstract address in an abstract address space. Contrast this with a physical address that refers to an actual physical address in memory. A logical address is generated by the CPU and is translated into a physical address by the memory management unit(MMU). Therefore, physical addresses are generated by the MMU

Can a multithreaded solution using multiple user-level threads achieve better performance on a multiprocessor system than on a single processor system? Explain.

A multithreaded system comprising of multiple user-level threads cannot make use of the different processors in a multiprocessor system simultaneously. The operating system sees only a single process and will not schedule the different threads of the process on separate processors. Consequently, there is no performance benefit associated with executing multiple user-level threads on a multiprocessor system.

Under what circumstances do page faults occur? Describe the actions taken by the operating system when a page fault occurs.

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 program if it is invalid. If it is valid, a free frame is located and I/O is requested to read the needed page into the free frame. Upon completion of the I/O, the process table and page table are updated and the instruction is restarted.

Show that, if the wait() and signal() semaphore operations are not executed atomically, then mutual exclusion may be violated.

A wait operation atomically decrements the value associated with a semaphore. If two wait operations are executed on a semaphore when its value is 1, if the two operations are not performed atomically, then it is possible that both operations might proceed to decrement the semaphore value, thereby violating mutual exclusion.

What is the main advantage of the layered approach to system design?What are the disadvantages of using the layered approach?

As in all cases of modular design, designing an operating system in a modular way has several advantages. The system is easier to debug and modify because changes affect only limited sections of the system rather than touching all sections of the operating system. Information is kept only where it is needed and is accessible only within a defined and restricted area, so any bugs affecting that data must be limited to a specific module or layer. One disadvantage is that the system is often slightly less efficient.

Race conditions are possible in many computer systems. Consider a banking system that maintains an account balance with two functions: deposit(amount) and withdraw(amount). These two functions are passed the amount that is to be deposited or withdrawn from the bank account balance. Assume that a husband and wife share a bank account. Concurrently, the husband calls the withdraw() function and the wife calls deposit(). Describe how a race condition is possible and what might be done to prevent the race condition from occurring.

Assume the balance in the account is 250.00 and the husband calls withdraw(50) and the wife calls deposit(100). Obviously the correct value should be 300.00 Since these two transactions will be serialized, the local value of balance for the husband becomes 200.00, but before he can commit the transaction, the deposit(100) operation takes place and updates the shared value of balance to 300.00. We then switch back to the husband and the value of the shared balance is set to 200.00 obviously an incorrect value. This problem can be avoided by locking.

The Linux kernel has a policy that a process cannot hold a spinlock while attempting to acquire a semaphore. Explain why this policy is in place.

Because acquiring a semaphore may put the process to sleep while it is waiting for the semaphore to become available. Spinlocks are to only be held for short durations and a process that is sleeping may hold the spinlock for too long a period.

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 means that a process is waiting for a condition to be satisfied in a tight loop without relinquishing the processor. Alternatively, a process could wait by relinquishing the processor, and block on a condition and wait to be awakened at some appropriate time in the future. Busy waiting can be avoided but incurs the overhead associated with putting a process to sleep and having to wake it up when the appropriate program state is reached.

What is the effect of allowing two entries in a page table to point to the same page frame in memory? Explain how this effect could be used to decrease the amount of time needed to copy a large amount of memory from one place to another. What effect would updating some byte on the one page have on the other page?

By allowing two entries in a page table to point to the same page frame in memory, users can share code and data. If the code is reentrant, much memory space can be saved through the shared use of large programs such as text editors, compilers, and database systems. "Copying" large amounts of memory could be effected by having different page tables point to the same memory location. However, sharing of nonreentrant code or data means that any user having access to the code can modify it and these modifications would be reflected in the other user's "copy."

One technique for implementing lottery scheduling works by assigning processes lottery tickets, which are used for allocating CPU time. Whenever a scheduling decision has to be made, a lottery ticket is chosen at random, and the process holding that ticket gets the CPU. The BTV operating system implements lottery scheduling by holding a lottery 50 times each second, with each lottery winner getting 20 milliseconds of CPU time (20 milliseconds × 50 = 1 second). Describe how the BTV scheduler can ensure that higher-priority threads receive more attention from the CPU than lower-priority threads.

By assigning more lottery tickets to higher-priority processes.

Give two reasons why caches are useful. What problems do they solve? What problems do they cause? If a cache can be made as large as the device for which it is caching (for instance, a cache as large as a disk), why not make it that large and eliminate the device?

Caches are useful when two or more components need to exchange data, and the components perform transfers at differing speeds. Caches solve the transfer problem by providing a buffer of intermediate speed between the components. If the fast device finds the data it needs in the cache, it need not wait for the slower device. The data in the cache must be kept consistent with the data in the components. If a component has a data value change, and the datum is also in the cache, the cache must also be updated. This is especially a problem on multiprocessor systems where more than one process may be accessing a datum.

Explain why Windows, Linux, and Solaris implement multiple locking mechanisms. Describe the circumstances under which they use spinlocks, mutex locks, semaphores, adaptive mutex locks, and condition variables. In each case, explain why the mechanism is needed.

Describe the circumstances under which they use spinlocks, mutex locks, semaphores, adaptive mutex locks, and condition variables. In each case, explain why the mechanism is needed. These operating systems provide different locking mechanisms depending on the application developers' needs. Spinlocks are useful for multiprocessor systems where a thread can run in a busy-loop (for a short period of time) rather than incurring the overhead of being put in a sleep queue. Mutexes are useful for locking resources. Solaris 2 uses adaptive mutexes, meaning that the mutex is implemented with a spin lock on multiprocessor machines. Semaphores and condition variables are more appropriate tools for synchronization when a resource must be held for a long period of time, since spinning is inefficient for a long duration.

Discuss the hardware support required to support demand paging.

For every memory-access operation, the page table needs to be consulted to check whether the corresponding page is resident or not and whether the program has read or write privileges for accessing the page. These checks have to be performed in hardware. A TLB could serve as a cache and improve the performance of the lookup operation.

Why is it important for the scheduler to distinguish I/O-bound programs from CPU-bound programs?

I/O-bound programs have the property of performing only a small amount of computation before performing I/O. Such programs typically do not use up their entire CPU quantum. CPU-bound programs, on the other hand, use their entire quantum without performing any blocking I/O operations. Consequently, one could make better use of the computer's resouces by giving higher priority to I/O-bound programs and allow them to execute ahead of the CPU-bound programs.

Describe the actions taken by a kernel to context-switch between processes.

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.

Explain why interrupt and dispatch latency times must be bounded in a hard real-time system.

Interrupt latency involves the following tasks: save the currently executing instruction, determine the type of interrupt, save the current process state, and then invoke the appropriate interrupt service routine. Dispatch latency is the cost associated with stopping one process and starting another. Both interrupt and dispatch latency needs to be minimized in order to ensure that real-time tasks receive immediate attention. Furthermore, sometimes interrupts are disabled when kernel data structures are being modified, so the interrupt does not get serviced immediately. For hard real-time systems, the time-period for which interrupts are disabled must be bounded in order to guarantee the desired quality of service.

Explain why interrupts are not appropriate for implementing synchronization primitives in multi processor systems.

Interrupts are not sufficient in multiprocessor systems since disabling interrupts only prevents other processes from executing on the processor in which interrupts were disabled; there are no limitations on what processes could be executing on other processors and therefore the process disabling interrupts cannot guarantee mutually exclusive access to program state.

What is the purpose of the command interpreter? Why is it usually separate from the kernel?

It reads commands from the user or from a file of commands and executes them, usually by turning them into one or more system calls.It is usually not part of the kernel since the command interpreter is subject to changes.

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?

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.

You have devised a new page-replacement algorithm that you think may be optimal. In some contorted test cases, Belady's anomaly occurs. Is the new algorithm optimal? Explain your answer.

No. An optimal algorithm will not suffer from Belady's anomaly because —by definition—an optimal algorithm replaces the page that will not be used for the longest time. Belady's anomaly occurs when a page replacement algorithm evicts a page that will be needed in the immediate future. An optimal algorithm would not have selected such a page.

In Chapter 3, we discussed Google's Chrome browser and its practice of opening each new website in a separate process. Would the same benefits have been achieved if instead Chrome had been designed to open each new website in a separate thread? Explain.

No. The primary reason for opening each website in a separate process is that if a web application in one website crashes, only that renderer process is affected, and the browser process, as well as other renderer processes, are unaffected. Because multiple threads all belong to the same process, any thread that crashes would affect the entire process.

When a process creates a new process using the fork() operation, which of the following is shared between the parent process and the child process? a. Stack b. Heap c. Shared memory segments

Only the shared memory segments are shared between the parent process and the newly forked child process. Copies of the stack and the heap are made for the newly created process.

Explain the difference between preemptive and nonpreemptive scheduling.

Preemptive scheduling allows a process to be interrupted in the midst of its execution, taking the CPU away and allocating it to another process. Nonpreemptive scheduling ensures that a process relinquishes control of the CPU only when it finishes with its current CPU burst.

What advantage is there in having different time-quantum sizes at different levels of a multilevel queueing system?

Processes that need more frequent servicing, for instance, interactive processes such as editors, can be in a queue with a small time quantum. Processes with no need for frequent servicing can be in a queue with a larger quantum, requiring fewer context switches to complete the processing, and thus making more efficient use of the computer.

Why are page sizes always powers of 2?

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.

Which of the following scheduling algorithms could result in starvation? a. First-come, first-served b. Shortest job first c. Round robin d. Priority

Shortest job first and priority-based scheduling algorithms could result in starvation.

Describe a mechanism by which one segment could belong to the address space of two different processes.

Since segment tables are a collection of base-limit registers, segments can be shared when entries in the segment table of two different jobs point to the same physical location. The two segment tables must have identical base pointers, and the shared segment number must be the same in the two processes.

I have stressed the need for an operating system to make efficient use of the computing hardware. When is it appropriate for the operating system to forsake this principle and to "waste" resources? Why is such a system not really wasteful?

Single-user systems should maximize use of the system for the user. A GUI might "waste" CPU cycles, but it optimizes the user's interaction with the system.

What is a true statement concerning open source operating systems is true?

Source code is freely available

Explain why spinlocks are not appropriate for single-processor systems yet are often used in multiprocessor systems.

Spinlocks are not appropriate for single-processor systems because the condition that would break a process out of the spinlock can be obtained only by executing a different process. If the process is not relinquishing the processor, other processes do not get the opportunity to set the program condition required for the first process to make progress. In a multiprocessor system, other processes execute on other processors and thereby modify the program state in order to release the first process from the spinlock.

What is the purpose of system calls?What is the purpose of system calls?

System calls allow user-level processes to request services of the operating system.

What is the purpose of system programs?

System programs can be thought of as bundles of useful system calls. They provide basic functionality to users so that users do not need to write their own programs to solve common problems.

The Sun UltraSPARC processor has multiple register sets. Describe what happens when a context switch occurs if the new context is already loaded into one of the register sets. What happens if the new context is in memory rather than in a register set and all the register sets are in use?

The CPU current-register-set pointer is changed to point to the set containing the new context, which takes very little time. If the context is in memory, one of the contexts in a register set must be chosen and be moved to memory, and the new context must be loaded from memory into the set. This process takes a little more time than on systems with one set of registers, depending on how are placement victim is selected.

explain the purpose of an interrupt vector

The interrupt vector is merely a table of pointers to specific interrupt-handling routines. Because there are a fixed number of interrupts, this table allows for more efficient handling of the interrupts than with a general-purpose, interrupt-processing routine.

What is the main difficulty that a programmer must overcome in writing an operating system for a real-time environment?

The main difficulty is keeping the operating system within the fixed time constraints of a real-time system. If the system does not complete a task in a certain time frame, it may cause a break down of the entire system it is running. Therefore when writing 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.

Consider a system in which a program can be separated into two parts: code and data. The CPU knows whether it wants an instruction (instruction fetch) or data (data fetch or store). Therefore, two base- limit register pairs are provided: one for instructions and one for data. The instruction base-limit register pair is automatically read-only, so programs can be shared among different users. Discuss the advantages and disadvantages of this scheme.

The major advantage of this scheme is that it is an effective mechanism for code and data sharing. For example, only one copy of an editor or a compiler needs to be kept in memory, and this code can be shared by all processes needing access to the editor or compiler code. Another advantage is protection of code against erroneous modification. The only disadvantage is that the code and data must be separated, which is usually adhered to in a compiler-generated code.

In Chapter 5, we discussed possible race conditions on various kernel data structures. Most scheduling algorithms maintain a run queue, which lists processes eligible to run on a processor. On multicore systems, there are two general options: (1)each processing core has its own run queue (2)a single run queue is shared by all processing cores. What are the advantages and disadvantages of each of these approaches?

The primary advantage of each processing core having its own run queue is that there is no contention over a single run queue when the scheduler is running concurrently on 2 or more processors. When a scheduling decision must be made for a processing core, the scheduler only need to look no further than its private run queue. A disadvantage of a single run queue is that it must be protected with locks to prevent a race condition and a processing core may be available to run a thread, yet it must first acquire the lock to retrieve the thread from the single queue. However, load balancing would likely not be an issue with a single run queue, whereas when each processing core has its own run queue, there must be some sort of load balancing between the different run queues.

The traditional UNIX scheduler enforces an inverse relationship between priority numbers and priorities: the higher the number, the lower the priority. The scheduler recalculates process priorities once per second using the following function: Priority = (recent CPU usage / 2) + base where base = 60 and recent CPU usage refers to a value indicating how often a process has used the CPU since priorities were last recalculated. Assume that recent CPU usage for process P1 is 40, for process P2 is 18, and for process P3 is 10. What will be the new priorities for these three processes when priorities are recalculated? Based on this information, does the traditional UNIX scheduler raise or lower the relative priority of a CPU-bound process?

The priorities assigned to the processes are 80, 69, and 65 respectively. The scheduler lowers the relative priority of CPU-bound processes.

Discuss ways in which the priority inversion problem could be addressed in a real-time system. Also discuss whether the solutions could be implemented within the context of a proportional share scheduler.

The priority inversion problem could be addressed by temporarily changing the priorities of the processes involved. Processes that are accessing resources needed by a higher-priority process inherit the higher priority until they are finished with the resources in question. When they are finished, their priority reverts to its original value. This solution can be easily implemented within a proportional share scheduler; the shares of the high-priority processes are simply transferred to the lower-priority process for the duration when it is accessing the resources.

Consider a system implementing multilevel queue scheduling. What strategy can a computer user employ to maximize the amount of CPU time allocated to the user's process?

The program could maximize the CPU time allocated to it by not fully utilizing its time quantums. It could use a large fraction of its assigned quantum, but relinquish the CPU before the end of the quantum, thereby increasing the priority associated with the process.

Assume that a distributed system is susceptible to server failure. What mechanisms would be required to guarantee the "exactly once" semantics for execution of RPCs?

The server should keep track in stable storage (such as a disk log) information regarding what RPC operations were received, whether they were successfully performed, and the results associated with the operations. When a server crash takes place and a RPC message is received, the server can check whether the RPC had been previously performed and therefore guarantee "exactly once" semanctics for the execution of RPCs.

Which of the following components of program state are shared across threads in a multithreaded process? a. Register values b. Heap memory c. Global variables d. Stack memory

The threads of a multithreaded process share heap memory and global variables. Each thread has its separate set of register values and a separate stack.

What are the three major activities of an operating system with regard to memory management?

The three major activities are:a. Keep track of which parts of memory are currently being used and by whom. b. Decide which processes are to be loaded into memory when memory space becomes available. c. Allocate and deallocate memory space as needed.

What are the three major activities of an operating system with regard to secondary-storage management?

The three major activities are:• Free-space management. • Storage allocation.• Disk scheduling.

Using Amdahl's Law, calculate the speedup gain of an application that has a 75 percent parallel component for: (a)two processing cores (b)four processing cores (c)thirty-two processing cores (d)A billion processing cores

Using Amdahl's Law, calculate the speedup gain of an application that has a 75 percent parallel component. Amdahl's is speedup ≤ 1/(S+ ((1 -S))/N) N = processing cores & S = % serial Two processing cores 1/(0.25 + ((1 - 0.25))/2) ≈ 1.6 speedup Four processing cores 1/(0.25 + ((1 - 0.25))/4) ≈ 2.28 speedup Thirty-two processing cores 1/(0.25 + ((1 -0.25))/32) ≈ 3.66 speedup A billion processing cores 1/(0.25 + ((1 -0.25))/1000000000) ≈ 4.00 speedup

Under what circumstances does a multithreaded solution using multiple kernel threads provide better performance than a single-threaded solution on a single-processor system?

When a kernel thread suffers a page fault, another kernel thread can be switched in to use the interleaving time in a useful manner. A single threaded process, on the other hand, will not be capable of performing useful work when a page fault takes place. Therefore, in scenarios where a program might suffer from frequent page faults or has to wait for other system events, a multithreaded solution would perform better even on a single-processor system.

Is it possible to have concurrency but not parallelism? Explain.

Yes. Concurrency means that more than one process or thread is progressing at the same time. However, it does not imply that the processes are running simultaneously. The scheduling of tasks allows for concurrency, but parallelism is supported only on systems with more than one processing core.

Describe the differences among short-term, medium-term, and long-term scheduling.

a. Short-term (CPU scheduler)—selects from jobs in memory those jobs that are ready to execute and allocates the CPU to them. b. Medium-term—used especially with time-sharing systems as an intermediate scheduling level. A swapping scheme is implemented to remove partially run programs from memory and reinstate them later to continue where they left off. c. Long-term (job scheduler)—determines which jobs are brought into memory for processing. The primary difference is in the frequency of their execution. The short term must select a new process quite often. Long-term is used much less often since it handles placing jobs in the system and may wait a while for a job to finish before it admits another one.

Assume that you have a page-reference string for a process with m frames (initially all empty). The page-reference string has length p; n distinct page numbers occur in it. Answer these questions for any page-replacement algorithms: a.What is a lower bound on the number of page faults? b.What is an upper bound on the number of page faults?

a. n (just one fault for each page as it is initially loaded) b. p (a fault on every entry in the page-reference string)

Provide two programming examples in which multithreading does not provide better performance than a single-threaded solution.

a.Any kind of sequential program is not a good candidate to be threaded. An example of this is a program that calculates an individual tax return. b.Another example is a "shell" program such as the C-shell or Korn shell. Such a program must closely monitor its own working space such as open files, environment variables, and current working directory.

Discuss how the following pairs of scheduling criteria conflict in certain settings. a. CPU utilization and response time b. Average turnaround time and maximum waiting time c. I/O device utilization and CPU utilization

a.CPU utilization and response time: CPU utilization is increased if the overheads associated with context switching is minimized. The context switching over heads could be lowered by performing context switches infrequently. This could, however, result in increasing the response time for processes. Average turnaround time and maximum waiting time: Average turnaround time is minimized by executing the shortest tasks first. Such a scheduling policy could, however, starve long-running tasks and thereby increase their waiting time. I/O device utilization and CPU utilization: CPU utilization is maximized by running long-running CPU-bound tasks without performing context switches. I/O device utilization is maximized by scheduling I/O-bound jobs as soon as they become ready to run, thereby incurring the overheads of context switches

Explain the differences in how much the following scheduling algorithms discriminate in favor of short processes: a.FCFS b.RR c.Multilevel feedback queues

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.

Consider a variant of the RR scheduling algorithm where the entries in the ready queue are pointers to the PCBs. a.What would be the effect of putting two pointers to the same process in the ready queue? b.What would be two major advantages and disadvantages of this scheme? c.How would you modify the basic RR algorithm to achieve the same effect without the duplicate pointers?

a.In effect, that process will have increased its priority since by getting time more often it is receiving preferential treatment. b.The advantage is that more important jobs could be given more time, in other words, higher priority in treatment. The consequence, of course, is that shorter jobs will suffer. c.Allot a longer amount of time to processes deserving higher priority. In other words, have two or more quantums possible in the Round-Robin scheme.

Consider a logical address space of 64 pages of 1024 words each, mapped onto a physical memory of 32 frames. a. How many bits are there in the logical address? b. How many bits are there in the physical address?

a.Logical address: 16 bits b.Physical address: 15 bits

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

a.Thrashing is occurring. b.CPU utilization is sufficiently high to leave things alone. c.Increase the degree of multiprogramming.

The list of processes waiting for a particular IO device is called a(n)

device queue

the ____ multithreading model multiplexes map many user-level threads to a small or equal number of kernel threads

many-to-many

A CPU-scheduling algorithm determines an order for the execution of its scheduled processes. Given n processes to be scheduled on one processor, how many different schedules are possible? Give a formula in terms of n.

n! [n factorial = n×(n -1)×(n -2)×...×2×1]

What are the three main purposes of an OS?

• 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 (2) management of the operation and control of I/O devices.