CS 416 Exam 1
Spring 2012
See the solutions (6 per page).
- 4 points
UNIX-derived systems execute new programs via a two-step process of fork and execve. Other systems provide a single system call to run a new program. Explain an advantage of using this two-step approach. - 4 points
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What is the potential danger in running this code?
while (1) fork(); - What should be done in the kernel to limit this problem?
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- 4 points
We need to sum up a huge array of numbers and will use multiple threads to do so. A worker thread is started with a thread ID as a parameter, where threads are numbered 0, 1, ... (numvalues-1). Each of N worker threads sums up every Nth element of the array in parallel into a local variable, sum. At the end, they each add their answer to a running total called total. A separate thread, main, dispatches the workers and computes and prints the final result.
/* global values */ int total, values[], numvalues, N; sem s= event e= main() { pthread t[N]; int i; for (i=0; i < N; ++i) { pthread_create(&t[i], NULL, worker, i); } printf(“total = %d\n”, total); } worker(int i) { int j, sum=0; for (j=i; j < numvalues; j+=N) { sum += values[j]; } total += sum; }Using one semaphore, s, and one event variable e, and without waiting on (joining) threads, define the initial values of s and e and the placement of up(s)/down(s) and advance(e)/await(e, value) operations to ensure that all the summations takes place without race conditions and only when the data is ready.
- 4 points
What is the difference between deadlock and starvation? - 4 points
If short time slices improve interactive performance, what is the downside? - 4 points
How does Multilevel Feedback Queue scheduling try to approximate Shortest Remaining Time First (SRTF) scheduling? How does it not have the disadvantage of SRTF? - In contrast to the BIOS, EFI (Extensible Firmware Interface):
(a) Is stored in system firmware instead of on the disk.
(b) Loads a boot loader from the master boot record (MBR).
(c) Bypasses the Master Boot Record (MBR) and load a boot loader from the volume boot record (VBR) directly.
(d) Can load a boot loader from a file system on the disk. - Which of the following is most likely to be a system call?
(a) The implementation of a while loop in C.
(b) Parse a token from a string.
(c) Get the cosine of a number.
(d) Get the time of day. - The primary use of a Programmable Interval Timer (PIT) is to:
(a) Measure the elapsed time between two events.
(b) Put the system to sleep but have it wake up later at a predetermined time.
(c) Put a process to sleep for a specified interval.
(d) Make sure that the kernel gets control of the processor periodically. - Which of these statements is true?
(a) A mode switch precedes a context switch.
(b) A context switch precedes a mode switch.
(c) A context switch can occur without a mode switch.
(d) A mode switch is just a different name for a context switch. - Disk controllers tend to use Direct Memory Access (DMA) over Programmed I/O (PIO) because:
(a) The CPU does not have to copy the disk data byte by byte.
(b) Most disks are not programmable.
(c) Transferring data to or from a disk is performed by the kernel.
(d) The entire disk appears as memory to an operating system. - The first demonstration of a computer system with a graphical display, keyboard, mouse, networking, shared screen collaboration, and hypertext took place in:
(a) 1956
(b) 1968
(c) 1980
(d) 1992 - Which process state transition is not valid?
(a) Ready → Running
(b) Running → Ready
(c) Running → Blocked
(d) Blocked → Running - Which process state transition is valid only on a preemptive multitasking system?
(a) Ready → Running
(b) Running → Ready
(c) Running → Blocked
(d) Blocked → Running - A Thread Control Block (PCB) stores:
(a) User (owner) ID
(b) Memory map
(c) The machine state (registers, program counter)
(d) Open file descriptors - A race condition is:
(a) When one process is trying to beat another to execute a region of code.
(b) When a process cannot make progress because another one is blocking it.
(c) When the outcome of processes is dependent on the exact order of execution among them.
(d) A form of locking where processes coordinate for exclusive access to a critical section. - A process scheduler is responsible for moving processes between these states:
(a) Ready and Blocked
(b) Running and Blocked
(c) Ready and Running
(d) Ready, Running, and Blocked - The wait system call on UNIX systems puts a process to sleep until:
(a) A semaphore wakes it up.
(b) The specified elapsed time expires.
(c) A child process terminates.
(d) The process is preempted by another process. - Priority inversion occurs when:
(a) A scheduler repeatedly schedules a low-priority thread over a high-priority one.
(b) A high-priority thread is blocked, causing the scheduler to run a low-priority thread.
(c) A scheduler schedules a high-priority thread when it would be better to schedule a low-priority one.
(d) A high-priority thread wakes up a low-priority thread, causing it to be scheduled. - Every process gets the same share of the CPU with a:
(a) Round-robin scheduler.
(b) Shortest remaining time first scheduler.
(c) Priority scheduler.
(d) Multilevel feedback queues. - A weighted exponential average is useful in process scheduling for:
(a) Determining the length of a quantum for the thread.
(b) Allowing a priority scheduler to assign a priority to the process.
(c) Estimating the length of the next CPU burst for the thread.
(d) Ordering processes in the ready queue for a round robin scheduler. - Process aging:
(a) Increases the priority of a process if it sits in the ready state for a long time.
(b) Decrease the priority of a process each time the process gets to run.
(c) Increases the priority of a process as it gets older.
(d) Decreases the priority of a process as it gets older - Which scheduler does not risk starvation of processes?
(a) Round-robin scheduler.
(b) Shortest remaining time first scheduler.
(c) Priority scheduler.
(d) Multilevel feedback queues. - Processes A and B are ready to run at the same time. A needs 600 msec of CPU time. B needs 800 msec of CPU time. Both need to finish within one second. Assume that a Least Slack scheduler is used with an infinitesimally small time slice. When do A and B terminate?
(a) A at 600 msec, B at 1400 msec.
(b) A at 1400 msec, B at 800 msec.
(c) A and B at 1400 msec.
(d) A and B at 1000 msec (1 sec). - Now assume that an Earliest Deadline First scheduler is used. When do A and B terminate?
(a) A at 600 msec, B at 1400 msec.
(b) A at 1400 msec, B at 800 msec.
(c) A and B at 1400 msec.
(d) A and B at 1000 msec (1 sec). - Process A runs 30 times a second. Each run lasts for 4 msec (a total of 120 msec CPU per second).
Process B runs 60 times a second. Each run lasts for 2.5 msec (a total of 150 msec CPU per second).
Process C runs 4 times a second. Each run lasts for 45 msec (a total of 180 msec CPU per second).
Which is a valid priority assignment using Rata Monotonic assignment where higher numbers correspond with higher priorities?
(a) A=2, B=1, C=3
(b) A=2, B=3, C=1
(c) A=1, B=2, C=3
(d) A=3, B=2, C=1 - A system call always results in a context switch.
True False - On a pre-EFI PC architecture, the BIOS is loaded from the Master Boot Record (MBR), which then loads the Volume Boot Record (VBR)
True False - A process executes faster when running in kernel mode than when running in user mode.
True False - Hardware support for mutual exclusion, such as test-and-set locks has the advantage of avoiding the need for spin locks.
True False - Mailboxes are a form of messaging using indirect addressing.
True False - Rendezvous messaging between processes on the same system reduces the amount of message copying needed.
True False - Push migration may be used in multiprocessor scheduling with hard CPU affinity.
True False
Part I – 26 Points
PART II – 60 points – 3 points each
For each statement, select the most appropriate answer.
PART III – 14 points – 2 points each
For each statement, specify whether it is true or false by circling the correct choice.
