Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needsit

[Christoph Lameter]

On Tue, 27 Mar 2012, Steven Rostedt wrote:

> > Obviously compiling a kernel with preemptiong introduces additional
> > overhead to guarantee more deterministic behavior. Additional overhead
> > increases latencies generated by the OS in general. Compile a kernel
> > without preemption and it will run faster and thus have lower latencies.
>
> I call that "lower overhead".

Good marketing but it does not change the facts.

> "Latency is a measure of time delay experienced in a system, the precise
> definition of which depends on the system and the time being measured.
> Latencies may have different meaning in different contexts."
>
> That last sentence is key. So lets avoid the term "latency" as it
> obviously has a different meaning to the both of us.
>
> Instead, lets use "determinism" (what we call latency in the realtime
> world) and "overhead" (what you seem to see as latency caused by the
> kernel).

I sure wish you would be using the term determinism instead of "latency".

Overhead causes latency and the definition that you quoted is what I am
talking about. Latencies are the delays in processing experienced by the
application through the speed of system calls and by interruptions  of
a user space process by the kernel for various reasons.





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