multitasking

computer, parallel (Or "multi-tasking", "multi-processing",

"multiprogramming", "concurrency", "process scheduling") A

technique used in an operating system for sharing a single

processor between several independent jobs. The first

multitasking operating systems were designed in the early

1960s.

Under "cooperative multitasking" the running task decides

when to give up the CPU and under "pre-emptive multitasking"

(probably more common) a system process called the

"scheduler" suspends the currently running task after it has

run for a fixed period known as a "time-slice". In both

cases the scheduler is responsible for selecting the next task

to run and (re)starting it.

The running task may relinquish control voluntarily even in a

pre-emptive system if it is waiting for some external event.

In either system a task may be suspended prematurely if a

hardware interrupt occurs, especially if a higher priority

task was waiting for this event and has therefore become

runnable.

The scheduling algorithm used by the scheduler determines

which task will run next. Some common examples are

round-robin scheduling, priority scheduling, shortest jobfirst and guaranteed scheduling.

Multitasking introduces overheads because the processor

spends some time in choosing the next job to run and in saving

and restoring tasks' state, but it reduces the worst-case time

from job submission to completion compared with a simple

batch system where each job must finish before the next one

starts. Multitasking also means that while one task is

waiting for some external event, the CPU to do useful work

on other tasks.

A multitasking operating system should provide some degree of

protection of one task from another to prevent tasks from

interacting in unexpected ways such as accidentally modifying

the contents of each other's memory areas.

The jobs in a multitasking system may belong to one or many

users. This is distinct from parallel processing where one

user runs several tasks on several processors. Time-sharing

is almost synonymous but implies that there is more than one

user.

Multithreading is a kind of multitasking with low

overheads and no protection of tasks from each other, all

threads share the same memory.

(1998-04-24)