Preemptive and Nonpreemptive Kernels

Before moving to difference between preemptive kernels and nonpreemptive kernels we will discuss about basics.

Preemptive multitasking - Running several processes/threads on a single processor, creating the illusion that they run concurrently when actually each is allocated small multiplexed time slices to run in. A process is "preempted" when it is scheduled out of execution and waits for the next time slice to run in.

The main advantage of a preemptive kernel is that sys-calls do not block the entire system. If a sys-call takes a long time to finish then it doesn't mean the kernel can't do anything else in this time. The main disadvantage is that this introduces more complexity to the kernel code, having to handle more end-cases, perform more fine grained locking or use lock-less structures and algorithms.

Two general approaches are used to handle critical sections in operating systems: (1) preemptive kernels and (2) nonpreemptive kernels. 

Preemptive Kernel - A preemptive kernel is one that can be interrupted in the middle of executing code - for instance in response for a system call - to do other things and run other threads, possibly those that are not in the kernel.
In Kernel Mode : A preemptive kernel allows a process to be preempted while it is running in kernel mode. 

Nonpreemptive Kernel - A nonpreemptive kernel does not allow a process running in kernel mode to be preempted; a kernel-mode process will run until it exits kernel mode, blocks, or voluntarily yields control of the CPU. Obviously, a nonpreemptive kernel is essentially free from race conditions on kernel data structures, as only one process is active in the kernel at a time. We cannot say the same about nonpreemptive kernels, so they must be carefully designed to ensure that shared kernel data are free from race conditions. Preemptive kernels are especially difficult to design for 5MP architectures, since in these environments it is possible for two kernel-mode processes to run simultaneously on different processors.

Advantage of preemptive kernel over nonpreemptive kernel:
Why, then, would anyone favor a preemptive kernel over a nonpreemptive one? A preemptive kernel is more suitable for real-time programming, as it will allow a real-time process to preempt a process currently running in the kernel. Furthermore, a preemptive kernel may be more responsive, since there is less risk that a kernel-mode process will run for an arbitrarily long period before relinquishing the processor to waiting processes. Of course, this effect can be minimized by designing kernel code that does not behave in this way.
Usage of both kernels in OS :
Windows XP and Windows 2000 are nonpreemptive kernels, as is the traditional UNIX kernel. Prior to Linux 2.6, the Linux kernel was nonpreemptive as well. However, with the release of the 2.6 kernel, Linux changed to the preemptive model. Several commercial versions of UNIX are preemptive, including Solaris and IRIX.

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