CS 416 Exam 1
Spring 2014
See the solutions (6 per page).
- 3 points
How many times does this code print "hello"?
main(int argc, char **argv) { int i; for (i=0; i < 3; i++) { fork(); printf("hello\n"); } } - 3 points
How many times does this code print "hello"? Note that execl is a wrapper over the execve system call, which executes a program. In this case, the echo program is run.
main(int argc, char **argv) { int i; for (i=0; i < 3; i++) { execl("/bin/echo", "echo", "hello", 0); } } - 6 points
Your system supports messages but does not offer semaphores. Implement semaphore operations using messages. Assume that messages use a mailbox. You may assume a unique mailbox per semaphore (i.e., semaphore s corresponds to mailbox s). Sending a message is a non-blocking operation. Receiving a message is non-blocking only if there is a message ready to be read. Use C/Java stytle pseudocode.
Hint: you may send and receive empty messages (Ø). The three operations you have available are send(mbox, m) and recv(mbox, m), which respectively send a message m to mailbox mbox and receive a message m from mailbox mbox and new(mbox), which creates a mailbox mbox.
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Create a new semaphore s and initialize its value to N.
init_semaphore(s, N) -
up(s) -
down(s)
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Create a new semaphore s and initialize its value to N.
- Multiprogramming is:
(a) An executable program that is composed of modules built using different programming languages.
(b) Having multiple processors execute different programs at the same time.
(c) Keeping several programs in memory at once and switching between them.
(d) When one program has multiple threads that run concurrently. - With a legacy PC BIOS, the Master Boot Record:
(a) Identifies type of file system on the disk and loads the operating system.
(b) Contains the first code that is run by the computer when it boots up.
(c) Contains a list of operating systems available for booting.
(d) Contains a boot loader to load another boot loader located in the volume boot record. - Which of the following is a policy, not a mechanism?
(a) Create a thread.
(b) Prioritize processes that are using the graphics card.
(c) Send a message from one process to another.
(d) Delete a file. - Which of the following does NOT cause a trap?
(a) A user program divides a number by zero.
(b) The operating system kernel executes a privileged instruction.
(c) A programmable interval timer reaches its specified time.
(d) A user program executes an interrupt instruction. - A context switch always takes place when:
(a) The operating system saves the state of one process and loads another.
(b) A process makes a system call.
(c) A hardware interrupt takes place.
(d) A process makes a function call. - A dedicated system call instruction, such as SYSCALL, is:
(a) Faster than a software interrupt.
(b) More secure than a software interrupt.
(c) More flexible than a software interrupt.
(d) All of the above. - Which of the following is not a system call?
(a) Duplicate an open file descriptor.
(b) Get the current directory.
(c) Decrement a semaphore.
(d) Create a new linked list. - A process control block is:
(a) A structure that stores information about a single process.
(b) The kernel's structure for keeping track of all ready to run processes.
(c) A linked list of blocked processes (those waiting on some event).
(d) A kernel interface for controlling processes (creating, deleting, suspending). - A process exists in the zombie (also known as defunct) state because:
(a) It is running but making no progress.
(b) The user may need to restart it without reloading the program.
(c) The parent may need to read its exit status.
(d) The process may still have children that have not exited. - Which state transition is not valid?
(a) Ready → Blocked
(b) Running → Ready
(c) Ready → Running
(d) Running → Blocked - Threads within the same process do not share the same:
(a) Text segment (instructions).
(b) Data segment.
(c) Stack.
(d) Open files. - A race condition occurs when:
(a) Two or more threads compete to be the first to access a critical section.
(b) The outcome of a program depends on the specific order in which threads are scheduled.
(c) A thread grabs a lock for a critical section, thus preventing another thread from accessing it.
(d) Two threads run in lockstep synchronization with each other. - Which of the following techniques avoids the need for spinlocks?
(a) Event counters
(b) Test-and-set
(c) Compare-and-swap
(d) All of the above. - Priority inversion occurs when:
(a) A low priority thread has not been given a chance to run so its priority is temporarily increased.
(b) The scheduler allows a low priority process to run more frequently than a high priority process.
(c) Two or more threads are deadlocked and unable to make progress.
(d) A low priority thread is in a critical section that a high priority thread needs. - What's the biggest problem with spinlocks?
(a) They are vulnerable to race conditions.
(b) They are fundamentally buggy.
(c) They waste CPU resources.
(d) They rely on kernel support and cannot be implemented at user level. - A condition variable enables a thread to go to sleep and wake up when:
(a) The value of the variable is greater than or equal to some number N.
(b) Another thread sends a signal to that variable.
(c) Another thread increments the variable.
(d) Another thread reads the variable. - Preemption is when an operating system moves a process between these states:
(a) Running → Ready
(b) Running → Blocked
(c) Ready → Blocked
(d) Blocked → Running - The disadvantage of round-robin process scheduling is:
(a) It gives every process an equal share of the CPU.
(b) It can lead to starvation where some processes never get to run.
(c) It puts a high priority on interactive processes.
(d) It never preempts a process, so a long-running process holds everyone else up. - The downside to using a very small quantum is:
(a) A process might not get time to complete.
(b) The interactive performance of applications decreases.
(c) Some processes will not get a chance to run.
(d) Context switch overhead becomes significant. - A time-decayed exponential average of previous CPU bursts allows a scheduler to:
(a) Estimate when each process will complete execution and exit.
(b) Compute the optimum number of processes to have in the run queue.
(c) Pick the process that will be most likely block on I/O the soonest.
(d) Determine the overall load on the processor. - Process aging is when:
(a) A long-running process gets pushed to a lower priority level.
(b) A process that did not get to run for a long time gets a higher priority level.
(c) A long-running process gets pushed to a higher priority level.
(d) Memory and other resources are taken away from a process that has run for a long time. - The goal of a multilevel feedback queue is to:
(a) Keep the priority of interactive processes high.
(b) Gradually raise the priority of CPU-intensive processes.
(c) Ensure that each process gets the same share of the CPU regardless of how long it runs.
(d) Allow the scheduler to provide feedback to the process on how often it is being run. - With soft affinity on a multiprocessor system, the scheduler will:
(a) Try to use the same processor for the same process but move it if another processor has no work.
(b) Associate a process with a specific processor and ensure it always runs on that processor.
(c) Use a single run queue so that there is no ongoing association between processors and processes.
(d) Periodically reset the association between all processes and processors. - Process A has a deadline of 100 ms and requires 80 ms of compute time.
Process B has a deadline of 80 ms and requires 50 ms of compute time.
Process C a deadline of 50 ms and requires 10 ms of compute time.
In what order will a least slack scheduler schedule these processes?
(a) A, B, C
(b) C, B, A
(c) B, A, C
(d) C, A, B
Part I – 12 Points
PART II – 88 points – 4 points each
For each statement, select the most appropriate answer. I will give you credit for two incorrect answers.
