Preemptive multitasking lets an operating system interrupt a running process or thread and give the processor to another ready task. On one CPU core, this creates concurrency by alternating between tasks; multiple cores can also run separate threads at the same time.
How preemptive multitasking works
A task runs until a scheduling event gives the kernel a chance to reconsider which task should use the CPU. That event may be a timer interrupt, a higher-priority task becoming ready, or a system call that blocks while waiting for something such as input.
- The running task executes in user or kernel mode.
- A timer interrupt or another scheduling event transfers control to the kernel.
- The kernel saves the task’s execution state, including registers and the program counter, in data associated with the process or thread.
- The scheduler selects a ready task according to its policy and priorities, then restores that task’s saved state.
- The selected task resumes from the point where it stopped.
Microsoft describes the principle this way: “A multitasking operating system divides the available processor time among the processes or threads that need it.” Its Win32 documentation also explains that a thread receives a processor time slice and may be suspended when the slice expires: Microsoft Learn: Multitasking.
Time slices, priorities, and scheduling trade-offs
A time slice, also called a quantum, is the amount of CPU time a runnable task may receive before the scheduler can choose another. Microsoft uses approximately 20 milliseconds as an example, not as a universal setting; the actual slice depends on the operating system’s policy, processor, priority, and workload.
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Shorter slices can improve responsiveness because a waiting task gets another turn sooner. They also mean more frequent context switches. Longer slices reduce switching frequency and may help throughput, but can make interactive tasks wait longer. Scheduling is a balance among responsiveness, fairness, throughput, and overhead; policies and implementations vary by system. See Loyola University Chicago’s scheduling chapter.
Why switches have overhead
A context switch requires the kernel to save the outgoing task’s state, select another task, and restore its state. That work consumes CPU time. The newly scheduled task may also have less useful data in the processor cache and translation lookaside buffer (TLB), so it can take additional time to regain locality.
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The Loyola scheduling chapter illustrates the arithmetic with a 5 ms switching overhead: paired with a 20 ms quantum, overhead is 20%; paired with a 50 ms quantum, it is about 10%. These figures illustrate the trade-off in that example, not a benchmark for every operating system or computer.
Preemptive versus cooperative multitasking
The key difference is who can initiate a switch. In cooperative multitasking, a running program must yield voluntarily. In preemptive multitasking, the kernel can interrupt a task and schedule another, even if the first program does not choose to yield.
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| Aspect | Preemptive multitasking | Cooperative multitasking |
|---|---|---|
| Who initiates a switch? | The operating system can preempt a task in response to a timer or scheduling event. | The running application yields control voluntarily. |
| Protection against CPU monopolization | The kernel can limit how long a runnable task keeps the processor before reconsidering its choice. | A task that fails to yield can keep the CPU for too long. |
| Responsiveness when a task misbehaves | Other ready tasks can still be scheduled by the kernel. | Other work may be delayed until the task yields. |
| Switching overhead | Preemption and frequent switching have context-switch costs. | Voluntary switching can avoid some forced preemptions, though switching still has costs. |
| Typical use | Common in modern general-purpose operating systems and interactive workloads. | Found in some older operating systems and designs that rely on programs to cooperate. |
Examples of cooperative systems include CP/M, MS-DOS, Windows 1.x–3.x, classic Mac OS, and NetWare. Linux, BSD, Windows NT and later, macOS, VMS, and most UNIX systems use preemptive multitasking. These are broad platform-family examples; behavior can still differ by version and configuration. The Loyola scheduling overview discusses the distinction and examples.
Does preemptive multitasking mean tasks run simultaneously?
Not on a single CPU core: one core executes one instruction stream at a time. Preemption lets the operating system interleave tasks so rapidly that they appear to run at once. With multiple cores, separate threads can genuinely execute simultaneously on different cores. As Microsoft’s documentation notes, the operating system distributes processor time among work that needs it: Microsoft Learn: Multitasking.
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Key terms to know
- Process: A running program and the resources associated with it.
- Thread: A schedulable sequence of execution within a process.
- Ready task: A process or thread able to run if assigned a processor.
- Preemption: The kernel interrupting a running task so it can schedule another.
- Time slice (quantum): A period of CPU time allocated to a runnable task before the scheduler may choose another.
- Context switch: Saving one task’s execution state and restoring another’s.
Further reading
For a deeper treatment of processes, scheduling, and context switches, see the Operating Systems: Three Easy Pieces textbook.
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