Re: [PATCH] 2.4.0-prerelease: preemptive kernel.

[Nigel Gamble]

On Thu, 4 Jan 2001, ludovic fernandez wrote:
> This is not the point I was trying to make .....
> So far we are talking about real time behaviour. This is a very interesting/exciting
> thing and we all agree it's a huge task which goes much more behind
> just having a preemptive kernel.

You're right that it is more than just a preemptible kernel, but I don't
agree that it's all that huge.  But this is the third time I have worked
on enabling real-time behavior in unix-like OSes, so I may be biased ;-)

> I'm not convinced that a preemptive kernel is interesting for apps using
> the time sharing scheduling, mainly because it is not deterministic and the
> price of a mmu conntext switch is still way to heavy (that's my 2 cents belief
> anyway).

But as Roger pointed out, the number of extra context switches
introduced by having a preemptible kernel is actually very low.  If an
interrupt occurs while running in user mode, the context switch it may
cause will happen even in a non-preemptible kernel.  I think that
running a kernel compile for example, the number of context switches per
second caused by kernel preemption is probably between 1% and 10% of the
total context switches per second.  And it's certainly interesting to me
that I can listen to MP3s without interruption now, while doing a kernel
build!

Nigel Gamble                                    [e-mail]
Mountain View, CA, USA.                         http://www.nrg.org/