Messages in this thread | | | Date | Tue, 26 Feb 2002 19:01:01 -0800 | From | Andrew Morton <> | Subject | Re: low latency & preemtible kernels |
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george anzinger wrote: > > wwp wrote: > > > > Hi there, > > > > here's a newbie question: > > is it UNadvisable to apply both preempt-kernel-rml and low-latency patches > > over a 2.4.18 kernel? > > > > thanx in advance > > > > -- > I believe that the preempt kernel patch or one related to it does the > low-latency stuff in a more economical way,
Sigh. Not to single you out, George - I see abject misunderstanding and misinformation about this sort of thing all over the place.
So let's make some statements:
- preemption is more expensive that explicit scheduling points. Always was, always shall be.
- Anyone who has performed measurements knows that preemption is ineffective. Worst-case latencies are still up to 100 milliseconds.
- preemptability is a *basis* for getting a maintainable low-latency kernel. And that's the reason why I support its merge into 2.5. Same with Ingo, I expect.
But there's a lot of icky stuff to be done yet to make it effective.
> i.e. takes advantage of the > preemption code to implement the low-latency stuff. See the lock-break > patch that rml has. It should be right next to the preempt patch.
lock-break is missing the cross-SMP reschedule hack, so on SMP it'll still have very high worst-case latencies. If all the other parts of the low-latency patch were included then preempt+lock-break should give better results than low-latency.
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