multitasking
<computer, parallel> (Or "multi-tasking", "multiprogramming", "concurrent
processing", "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 job first 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)
Nearby terms:
Multistation Access Unit « multisync « Multisystem
eXtention Interface Bus « multitasking »
Multi-tasking Program for Microcomputers »
multithreaded » multithreading
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