What is the PREEMPT in the ssh message?

Question

When I'm login into the raspberry through ssh I'm receiving the following message

Linux raspberrypi 3.2.27+ #250 PREEMPT Thu Oct 18 19:03:02 BST 2012 armv6l

What does it mean the PREEMPT? Is that real time kernel?

Answer 1

PREEMPT seems to stand for "kernel preemption". This SO answer explain in a clear English what is a preemptive kernel:

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.

A preemptive kernel is one that can be interrupted in the middle of a executing code - for instance in response for a system call - to do other things and run other threads, possible that are not in the kernel. The main advantage in 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.

Answer 2

It means that you have preempted kernel version. More info about preempted kernel could be found here.

This option could be enabled while compiling kernel with option:

CONFIG_PREEMPT=y