Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

From: Andi Kleen
Date: Mon Jul 10 2017 - 13:27:28 EST

On Mon, Jul 10, 2017 at 06:42:06PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 10, 2017 at 07:46:09AM -0700, Andi Kleen wrote:
> > > So how much of the gain is simply due to skipping NOHZ? Mike used to
> > > carry a patch that would throttle NOHZ. And that is a _far_ smaller and
> > > simpler patch to do.
> >
> > Have you ever looked at a ftrace or PT trace of the idle entry?
> >
> > There's just too much stuff going on there. NOHZ is just the tip
> > of the iceberg.
> I have, and last time I did the actual poking at the LAPIC (to make NOHZ
> happen) was by far the slowest thing happening.

That must have been a long time ago because modern systems use TSC deadline
for a very long time ...

It's still slow, but not as slow as the LAPIC.

> Data to indicate what hurts how much would be a very good addition to
> the Changelogs. Clearly you have some, you really should have shared.