D686 Unit 6 questions and answers 2026

cycle

Repeating loop

CPU burst

Scheduling process state in which the process executes on CPU.

I/O burst

Scheduling process state in which the CPU performs I/O.

CPU scheduler

Kernel routine that selects a thread from the threads that are ready to execute and allocates a core to that thread.

nonpreemptive

Under nonpreemptive scheduling, once a core has been allocated to a thread the thread keeps the core until it releases the core either by terminating or by switching to the waiting state.

cooperative

A form of scheduling in which threads voluntarily move from the running state.

preemptive

A form of scheduling in which processes or threads are involuntarily moved from the running state (by for example a timer signaling the kernel to allow the next thread to run).

dispatcher

The dispatcher is the kernel routine that gives control of a core to the thread selected by the scheduler.

dispatch latency

The time it takes for the dispatcher to stop one thread and start another running.

CPU utilization

The percentage of time the CPU is actively working; the optimal utilization is around 90 percent, as sustained 100 percent use can impact system performance and stability.

turnaround time

The total time from process submission to completion, including waiting, execution, and I/O time.

waiting time

The time a process spends waiting in the ready queue before it gets CPU time.

response time

The time from the submission of a request to the first response being produced by the process, particularly important in interactive systems.

First-come first-served (FCFS)

The simplest scheduling algorithm - the thread that requests a core first is allocated the core first, and others following get cores in the order of their requests.

Gantt chart

A bar chart that is used in the text to illustrate a schedule.

convoy effect

A scheduling phenomenon in which threads wait for the one thread to get off a core, causing overall device and CPU utilization to be suboptimal.

shortest-job-first (SJF)

A scheduling algorithm that associates with each thread the length of the threads next CPU burst and schedules the shortest first.

exponential average

A calculation used in scheduling to estimate the next CPU burst time based on the previous burst times (with exponential decay on older values).

shortest-remaining-time-first (SJRF)

Similar to SJF, this scheduling algorithm optimizes for the shortest remaining time until thread completion.

round-robin (RR)

A scheduling algorithm that is designed especially for time-sharing systems - similar to FCFS scheduling, but preemption is added to enable the system to switch between threads.

time quantum

A small unit of time used by scheduling algorithms as a basis for determining when to preempt a thread from the CPU to allow another to run.

time slice

See time quantum.

priority-scheduling

A scheduling algorithm in which a priority is associated with each thread and the free CPU core is allocated to the thread with the highest priority.

infinite blocking

See starvation.

starvation

A scheduling risk in which a thread that is ready to run never gets put onto the CPU due to the scheduling algorithm - it is starved for CPU time.

aging

Aging is a solution to scheduling starvation and involves gradually increasing the priority of threads as they wait for CPU time.

multilevel queue

A multilevel queue scheduling algorithm partitions the ready queue into several separate queues.

foreground

A thread that is interactive and has input directed to it (such as a window currently selected as active or a terminal window that is currently selected to receive input).

background

A thread that is not currently interactive (has no interactive input directed to it) such as one in a batch job or not currently being used by a user.

multilevel feedback queue

The multilevel feedback queue scheduling algorithm that allows a process to move between queues.

real-time

systems where tasks must be completed within specific time limits

hard real-time systems

systems where tasks must meet deadlines without exception

soft real-time systems

systems that are not critical when tasks are important but missing a deadline

real-time scheduling

the process of organizing tasks so they meet their deadlines

online scheduler

a scheduler that makes decisions about tasks while the system is running

offline scheduler

a scheduler that plans tasks before the system starts running

static scheduler

a scheduler that makes a fixed schedule before tasks start

feasibility tests/schedulability tests

methods to check if tasks in a system can be completed on time

dynamic scheduler

a scheduler that adjusts the task schedule based on current conditions

preemptive scheduler

scheduler that can interrupt a currently running task to start or resume another task, ensuring that higher-priority tasks receive CPU time as needed

non-preemptive scheduler

a scheduler that allows a task to run to completion before switching to another task, ensuring that once a task starts, it is not interrupted until it finishes

multilevel queue

a scheduling algorithm dividing the ready queue into multiple distinct queues

foreground

refers to an interactive thread actively receiving input or engaging with user interaction

background

describes a thread or process not actively receiving user input or interaction, often running in batch mode or idle

multilevel feedback queue

a scheduling algorithm allowing processes to move between different priority queues based on their CPU usage characteristics

symmetric multiprocessing (SMP)

each processor manages its own scheduling, handling both kernel and user threads with potential contention for system resources

asymmetric multiprocessing (AMP)

A system where one processor handles all system tasks and scheduling, while other processors execute only user code

chip multithreading (CMT)

CPUs with multiple cores, each supporting numerous hardware threads, that enhance overall processing efficiency

load balancing

distributing workload evenly across processors in an SMP system to maximize efficiency and prevent idle processors

push migration

load-balancing technique where a task redistributes threads from overloaded processors to those with lighter loads

pull migration

load-balancing method where an idle processor retrieves tasks from busy processors to balance the workload

processor affinity

keeping a thread on the same processor to benefit from its cache and reduce cache invalidation costs

soft affinity

strategy where the operating system aims to keep a thread on the same processor but allows it to migrate if necessary

hard affinity

A strategy where the operating system allows a thread to specify a set of processors on which it can run, ensuring it stays within this set

memory stall

a delay in thread execution when accessing memory that is not currently in the CPU cache, requiring retrieval from main memory

hardware threads

threads that a CPU core can manage, either one per core or multiple, to optimize performance by switching threads during stalls