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multitasking 音標拼音: [m'ʌltit ,æskɪŋ] 多任務,多任務化 多任務,多任務化 multitasking多重任務;多重作業;多任務;多(重)任務處理 MTmulti- tasking多任務
multitasking多任務 (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)
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