From Wikipedia, the free encyclopedia

In computing, an idle task is a special task loaded by the OS scheduler only when there is nothing for the computer to do (that is, no other tasks are in the state of "running"). The idle task can be hard-coded into the scheduler, or it can be implemented as a separate task with the lowest possible priority. An advantage of the latter approach is that programs monitoring the system status can see the idle task along with all other tasks; an example is Windows NT's System idle process.

On modern processors, where a "HLT" (halt) instruction (which does nothing until the processor is interrupted) saves significant amounts of power and heat, the idle task almost always consists of a loop which repeatedly executes HLT instructions. However, on older computers, where temperature dissipation was almost constant with CPU load, the program would often do useless things, like blink the front panel lights in an amusing or recognizable pattern.

Often, this had the effect on timeshared systems that if one was lucky enough to have access to the computer room, one could glance at the front panel lights to see how busy the machine was. If the idle pattern very rarely showed up, the machine was heavily loaded, and one might go for lunch before waiting for a job to finish; on the other hand, if it was clearly blinking the idle pattern, one might run the job immediately.

In Unix-like operating systems such as Linux, the idle task has process ID zero, and never exits. Another specially distinguished task on Unix-like operating systems is the init process, which does little more than wait around for its child processes to die.