linux-kernel - Re: Scheduler regression: Too frequent timer interrupts(?)

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Message-ID: <alpine.DEB.1.10.0904281719480.15947@qirst.com>
Date:	Tue, 28 Apr 2009 17:21:29 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Pavel Machek <pavel@....cz>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: Scheduler regression: Too frequent timer interrupts(?)

On Tue, 28 Apr 2009, Peter Zijlstra wrote:

> On Tue, 2009-04-28 at 17:02 -0400, Christoph Lameter wrote:
> > On Thu, 23 Apr 2009, Pavel Machek wrote:
> >
> > > I'd say that taking interrupt and returning in below 1000nsec is
> > > impossible, even on very fast hw.
> >
> > Hmmm.... Could it be that CONFIG_NO_HZ caused the timer interrupt to be
> > avoided in earlier releases? NO_HZ theory at least is to only schedule
> > interrupts when  necessary. So an interrupt is needed at the end of the
> > scheduling intervals and not at HZ frequency.
>
> No, NOHZ only works on idle cpus.

It is advertised to do more:

config NO_HZ
        bool "Tickless System (Dynamic Ticks)"
        depends on GENERIC_TIME && GENERIC_CLOCKEVENTS
        select TICK_ONESHOT
        help
          This option enables a tickless system: timer interrupts will
          only trigger on an as-needed basis both when the system is
          busy and when the system is idle.


So NO_HZ is really IDLE_NO_HZ. The system is not tickless.


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