Introduction

Terms

scheduler
The part of an operating system which assigns resources to processes, tasks, or threads.
dispatcher
The operating system component which transitions a process to the running state.

Remarks

Scheduling Levels

Terms

long term scheduler
The part of an operating system which places new tasks into the ready state.

Remarks

Preemptive vs. Nonpreemptive Scheduling

Terms

preemption
The operating system act of interrupting a running task, removing it from the run state, and placing it in the ready state.
preemptive
Having the capability of preempting running tasks.
nonpreemptive
Not having the capability of preempting running tasks.
quantum
The maximum run time allowed for a process before preemption.

Remarks

Priorities

Terms

priority
An index of importance which can be used to determine scheduling
static priority
A priority which is not automatically adjusted by the system.
dynamic priority
A priority which is adjusted automatically by the system according to task behavior and system loading.

Remarks

Scheduling Objectives

Remarks

Scheduling Criteria

Terms

I/O bound
A property of a process in which processor use is low, and I/O requests are high.
CPU bound
A property of a process in which processor use is high, and I/O requests are low.
batch process
A process which executes without user interaction.
interactive process
A process which requires user interaction while executing.

Remarks

Scheduling Algorithms

Terms

priority scheduling
A scheduling policy which schedules tasks in descending order of priority.
priority aging
A process which dynamically raises the priority of a task according to the time it has been waiting in the ready queue.
FIFO scheduling
A scheduling policy which schedules tasks in the same order in which they enter the ready state.
Round Robin scheduling
A scheduling policy which scheduleses tasks in a fixed circular order.
shortest process first scheduling
A scheduling policy which schedules tasks in ascending order of estimated processing time.
shortest remaining time scheduling
A scheduling policy which schedules tasks in ascending order of estimated remaining processing time.
fair share scheduling
A scheduling policy which assigns tasks to groups, and allocates a percentage of CPU time to each group.

Remarks

Deadline Scheduling

Terms

deadline scheduling
A scheduling algorithm which dispatches the process with the closest deadline. 

Remarks

Real Time Scheduling

Terms

real time system
A system which must respond in real time to changing conditions.
hard real time scheduling
A scheduling policy which ensures that deadlines are met.
soft real time scheduling
A scheduling policy which attempts to meet deadlines.
earliest deadline first scheduling
A deadline sheduling policy which dispatches the task with the earliest completion deadline.
laxity
The difference between the time until a tasks completion deadline and its remaining processing time requirement.
minimum laxity first scheduling
A deadline scheduling policy which dispatches the task with the least laxity.

Remarks

Java Thread Scheduling

Remarks

External Links

New York University Process Scheduling

List of Terms

scheduler dispatcher long term scheduler preemption
preemptive nonpreemptive quantum priority
static priority dynamic priority I/O bound CPU bound
batch process interactive process priority scheduling priority aging
FIFO scheduling Round Robin scheduling shortest process first scheduling shortest remaining time scheduling
fair share scheduling deadline scheduling real time system hard real time scheduling
soft real time scheduling earliest deadline first scheduling laxity minimum laxity first scheduling