Re: [ANNOUNCE] 3.8-rc6-nohz4

From: Ingo Molnar
Date: Mon Feb 11 2013 - 04:59:18 EST

* Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:

> > I'm worried about the proliferation of not easily separable
> > config options. We already have way too many timer and
> > scheduler options to begin with.
> Like Steve said, this is for overhead reasons. The syscall
> uses the slow path so that's ok. But we add a callback to
> every exception, irq entry/exit, scheduler sched switch,
> signal handling, user and kernel preemption point. This all
> could be lowered using static keys but even that doesn't make
> me feel comfortable with this idea.
> Moreover, for now this is going to be used only on extreme
> usecases such as real time and HPC. If we really have to merge
> this into an all-in-one nohz kconfig, I suggest we wait for
> the feature to mature a bit and prove that it can be useful
> further those specialized workloads, and also that we can
> ensure it's off-case overhead is not significant.

I have no problems with making it an option initially - as long
as the options are logically named and interconnected.

In terms of overhead, a big plus is the reduction in user-space
execution overhead. At HZ=1000 we easily have 0.5%-1.0% overhead
currently. That is a *lot* of overhead if the box does mostly
user-space execution - which most boxes do, both servers and
desktops - not HPC systems.


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