Re: RT patch acceptance

[Ingo Molnar <mingo@elte.hu>]

* Andi Kleen <ak@muc.de> wrote:

> [...] Even normal kernels must have reasonably good latency, as long 
> as it doesnt cost unnecessary performance.

they do get reasonably good latency (within the hard constraints of the 
possibilities of a given preemption model), due to the cross-effects 
between the various preemption models, that i explained in detail in 
earlier mails. Something that directly improves latencies on 
CONFIG_PREEMPT improves the 'subsystem-use latencies' on PREEMPT_RT.  
Also there's the positive interaction between scalability and latencies 
as well.

but it's certainly not for free. Just like there's no zero-cost
virtualization, or there's no zero-cost nanokernel approach either,
there's no zero-cost single-kernel-image deterministic system either.

and the argument about binary kernels - that's a choice up to vendors
and users. Right now PREEMPT_NONE is dominant, so do you argue that
CONFIG_PREEMPT should be removed? It's certainly not zero-cost even on
the source code, witness all the preempt_disable()/preempt_enable() or
get_cpu()/put_cpu() uses.

	Ingo