In
computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
for
Operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
s, aging (US English) or ageing is a
scheduling
A schedule or a timetable, as a basic time-management tool, consists of a list of times at which possible task (project management), tasks, events, or actions are intended to take place, or of a sequence of events in the chronological order ...
technique used to avoid
starvation
Starvation is a severe deficiency in caloric energy intake, below the level needed to maintain an organism's life. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, dea ...
.
Fixed priority scheduling is a scheduling discipline, in which
tasks queued for utilizing a system resource are assigned a priority each. A task with a high priority is allowed to access a specific system resource before a task with a lower priority is allowed to do the same. A disadvantage of this approach is that tasks assigned with a lower priority may be starved when a large number of high priority tasks are queued. Aging is used to gradually increase the
priority of a task, based on its waiting time in the
ready queue In a multitasking computer system, processes may occupy a variety of states. These distinct states may not be recognized as such by the operating system kernel. However, they are a useful abstraction for the understanding of processes.
Primary ...
.
Problem
In priority-based
scheduling algorithms
In computing, scheduling is the action of assigning ''resources'' to perform ''tasks''. The ''resources'' may be central processing unit, processors, telecommunications link, network links or expansion cards. The ''tasks'' may be thread (computer ...
, a major problem is indefinite block, or
starvation
Starvation is a severe deficiency in caloric energy intake, below the level needed to maintain an organism's life. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, dea ...
. A process that is ready to run but waiting for the CPU can be considered blocked. A priority scheduling algorithm can leave some low-priority processes waiting indefinitely. A steady stream of higher-priority processes can prevent a low-priority process from ever getting the CPU.
Application
Aging is used to ensure that jobs with lower priority will eventually complete their execution. This technique can be used to reduce
starvation
Starvation is a severe deficiency in caloric energy intake, below the level needed to maintain an organism's life. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, dea ...
of low priority tasks.
Processor Scheduling – Notes – Operating Systems – Computer Science Now
/ref>
There are many ways to implement aging, but all have the same principle that the priority of a process should increase as it waits in the ready queue. The increase in priority may or may not be equal to the waiting time of the process.
Example
Suppose a system with priority range of 0-512. In this system, 0 means highest priority.
Consider a process with priority 127. If we increase its priority by 1 every 15 minutes, then in more than 32 hours the process will age to 0 priority and get executed.
References
Processor scheduling algorithms
{{operating-system-stub
^ Silberschatz, Galvin, Gagne Operating System Principles, 6th ed., p.162