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What is the difference between a "Non-preemptive", "Preemptive" and "Selective Preemptive" Kernel?

Hope someone can shed some light into this.

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On a preemptive kernel, a process running in kernel mode can be replaced by another process while in the middle of a kernel function.

This only applies to processes running in kernel mode, a CPU executing processes in user mode is considered "idle". If a user mode process wants to request a service from the kernel, he has to issue an exception which the kernel can handle.

As an example:

Process A executes an exception handler, Process B gets awaken by an IRQ request, the kernel replaces process A with B (a forced process switch). Process A is left unfinished. The scheduler decides afterwards if process A gets CPU time or not.

On a nonpreemptive kernel, process A would just have used all the processor time until he is finished or voluntarily decides to allow other processes to interrupt him (a planned process switch).

Today's Linux based operating systems generally do not include a fully preemptive kernel, there are still critical functions which have to run without interruption. So I think you could call this a "selective preemptive kernel".

Apart from that, there are approaches to make the Linux kernel (nearly) fully preemptive.

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The Macintosh System 6 and related code (e.g., System 7, System 8, and System 9) were all non-preemptive. Apple's switch to MacOS X also saw the introduction of preemptive multitasking as well. –  Mei Feb 4 '12 at 1:25
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the preemption is -> The ability of the operating system to preempt or stop a currently scheduled task in favour of a higher priority task. The scheduling may be one of, but not limited to, process or I/O scheduling etc.

Under Linux, user-space programs have always been preemptible : the kernel interrupts user-space programs to switch to other threads, using the regular clock tick. So, the kernel doesn't wait for user-space programs to explicitly release the processor (which is the case in cooperative multitasking). This means that an infinite loop in an user-space program cannot block the system.

However, until 2.6 kernels, the kernel itself was not preemtible : as soon as one thread has entered the kernel, it could not be preempted to execute an other thread. However, this absence of preemption in the kernel caused several problems with regard to latency and scalability. So, kernel preemption has been introduced in 2.6 kernels, and one can enable or disable it using the CONFIG_PREEMPT option. If CONFIG_PREEMPT is enabled, then kernel code can be preempted everywhere, except when the code has disabled local interrupts. An infinite loop in the code can no longer block the entire system. If CONFIG_PREEMPT is disabled, then the 2.4 behaviour is restored.

ReQuoted and formatted from: http://www.linuxquestions.org/questions/linux-general-1/pre-emptive-vs-non-pre-emptive-kernel-582437/

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Why was 2.4 kernel non-preemptive? –  Sen Dec 28 '10 at 10:04
    
Better ask Linus Torvalds :) –  Madhur Ahuja Dec 28 '10 at 10:18
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