Idle
| Depends on | Process States • Process Scheduling |
|---|
In a simple, single-tasking system, when the processor cannot do useful work it will simply ‘busy-wait’ until some stimulus (in the one job it’s currently doing) occurs.
In a multitasking system system it is usual to block the process and schedule something else.
What if there are no tasks in the ready state? The processor cannot (permanently) ‘halt’ because something is liable to want to run again in future.
It is normal to provide some sort of “idle” task, which busily does nothing except wait until a ‘real’ process becomes ready. In modern processors there is often a ‘SLEEP’ type instruction which puts the processor into a low-power mode until (for example) an interrupt is requested, since only an external event is going to cause a change in processor state.
The idle process will probably have to be a bit ‘special’, depending on the scheduling algorithm.
- In a real-time system (where, by definition, all other processes will block on occasion) it may be sufficient to simply run
idlecontinuously at a uniquely low priority. - In an interactive system it is undesirable for
idleto have regularly scheduled time-slices as this can detract from the time available for useful processing.
The idle process may be a bit smarter than purely ‘do
nothing’; for example it could note the times it starts and
stops for ‘accounting’ purposes.
(You may be able to find this in (e.g.) the Unix top utility.)
