Re: The "best" value of HZ

[Con Kolivas <kernel@kolivas.org>]

On Fri, 28 Oct 2005 02:00 pm, Jesper Juhl wrote:
> On 10/28/05, Lee Revell <rlrevell@joe-job.com> wrote:
> > On Fri, 2005-10-28 at 03:31 +0100, Alistair John Strachan wrote:
> > > On Friday 28 October 2005 00:18, Claudio Scordino wrote:
> > > > Hi,
> > > >
> > > >     during the last years there has been a lot of discussion about
> > > > the "best" value of HZ... On i386 was 100, then became 1000, and
> > > > finally was set to 250. I'm thinking to do an evaluation of this
> > > > parameter using different architectures.
> > > >
> > > > Has anybody thought to give the possibility to modify the value of HZ
> > > > at boot time instead of at compile time ? This would allow to easily
> > > > test different values on different machines and create a table
> > > > containing the "best" value for each architecture...  At this moment,
> > > > instead, we have to recompile the kernel for each different value :(
> > > >
> > > > Do you think there would be much work to do that ?
> > > > Do you think it would be a desired feature the knowledge of the best
> > > > value for each architecture with more precision ?
> > >
> > > Google for "dynticks". There's obviously an overhead associated with HZ
> > > not being a constant (the compiler cannot optimise many expressions),
> > > but the feature is being worked on nonetheless.
> >
> > Well Linus had the best idea in that thread (as usual) which was to
> > implement "dynamic ticks" by leaving HZ a constant, setting it to a high
> > value, and skipping ticks when idle.  Has there been any work in that
> > direction?
>
> i did a bit of work in that area, but the stuff I came up with never
> seemed to work right, so I dropped it.

It's all still in development at the moment but not far from being available 
again. We stood back a bit to make some structural changes before trying to 
make it ready for prime time.

Cheers,
Con