linux-kernel - Re: [RFC] HZ free ntp

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Message-ID: <Pine.LNX.4.64.0612132125450.1867@scrub.home>
Date:	Wed, 13 Dec 2006 21:40:34 +0100 (CET)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	john stultz <johnstul@...ibm.com>
cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] HZ free ntp

Hi,

On Wed, 13 Dec 2006, john stultz wrote:

> > The largest possible interval is freq cycles (or 1 second without
> > adjustments). That is the base interval and without redesigning NTP we
> > can't change that. This base interval can be subdivided into smaller
> > intervals for incremental updates.
> 
> Indeed, larger then 1 second intervals would require the second_overflow
> code to be reworked too.

There isn't much to rework without a complete redesign.

> > You cannot choose arbitrary intervals otherwise you get other problems,
> > e.g. with your patch time_offset handling is broken.
> 
> I'm not seeing this yet. Any more details? 

time_offset is scaled to HZ in do_adjtimex, which needs to be changed as 
well.

> > You don't have to introduce anything new, it's tick_length that changes
> > and HZ that becomes a variable in this function.
> 
> So, forgive me for rehashing this, but it seems we're cross talking
> again. The context here is the dynticks code. Where HZ doesn't change,
> but we get interrupts at much reduced rates.

I know and all you have to change in the ntp and some related code is to 
replace HZ there with a variable, thus make it changable, so you can 
increase the update interval (i.e. it becomes 1s/hz instead of 1s/HZ).

> However, in doing so we have to
> work w/ the ntp.c code which (as Ingo earlier mentioned) has a number of
> HZ based assumptions.

Repeating Ingo's nonsense doesn't make it any more true. :-(

bye, Roman
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