Scheduling: Exam Questions

• Which term reflects a guaranteed response time? (One correct answer.)

  • Multithreading

  • Realtime

  • Scheduling

• What is the difference between a thread and a process? (One correct answer.)

  • A process has its own address space, where a thread has not.

  • A thread has its own address space, where a process has not.

• Which of the following statements about a process’s address space is true? (Wrong answers are subtracted from correct answers.)

  • A range of memory set aside for a process’s exclusive use.

  • Provides memory isolation between processes.

  • A pool of IP addresses that are made available to it.

  • A process cannot access memory outside of that space.

• What is the effect of running the command sha1sum /dev/zero? (Wrong answers are subtracted from correct answers.)

  • It consumes as much CPU time as it can.

  • It will exhaust all physical memory that is available.

  • It tries to calculate a SHA1 fingerprint of an infinite sequence of null bytes.

  • It calculates the sum of the major numbers of all character devices on the machine.

  • The process is compute-bound.

• Which of the following statements about fair scheduling are true? (Wrong answers are subtracted from correct answers.)

  • It is the default scheduling policy for a task (that task is said to be in the fair scheduling domain).

  • A task that is in the fair scheduling domain can provide guaranteed response times.

  • The scheduler favors IO bound processes over CPU bound processes.

  • Fair scheduling can be considered the opposite of realtime scheduling.

• Which of the following statements about the OS scheduler are true? (Wrong answers are subtracted from correct answers.)

  • The scheduler provides the illusion of task parallelism, even on single-CPU machines.

  • The scheduler is a core component of the GNOME desktop environment which manages the user’s to-do list, using machine learning.

  • Its main responsiblity is to assign CPU time slices to waiting tasks (effectively moving them into the “running” state).

  • Its main responsiblity is to assign CPU time slices to runnable tasks (effectively moving them into the “running” state).

• When a task becomes runnable, under which circumstances is it made running (i.e., is assigned a CPU to run on) immediately? (Wrong answers are subtracted from correct answers.)

  • When no task with higher priority is runnable.

  • When a CPU is free.

  • When all other tasks explicitly agree.

  • When the administrator permits it to.

• How does a CPU become available/free for another task to run on? (Wrong answers are subtracted from correct answers.)

  • By preemption: the scheduler interrupts a running task in favor of another (runnable) task.

  • Voluntarily: the task puts itself to sleep by declaring to the kernel that it has to wait for something.

  • The CPU is signaled (by an inter-processor interrupt) by another CPU’s cache that it should free resources. Reason: one task constantly thrashes the signaler’s cache.

• Which of the following statements about priority inversion is true? (Wrong answers are subtracted from correct answers.)

  • Priority inversion can occur only when fair scheduling is in place.

  • It is a deadlock scenario (a bug).

  • Realtime systems are particularly susceptible to priority inversion.

• Which of the following statements about wakeup latency is true? (Wrong answers are subtracted from correct answers.)

  • Wakeup latency is the time between the occurence of an event (usually an interrupt), and the point where a task that waits for that event enters the running state.

  • Realtime scheduling provides a guaranteed upper bound to wakeup latency.

  • Fair scheduling provides a guaranteed upper bound to wakeup latency.