linux-kernel - Re: [PATCH] x86: Reduce the default HZ value

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Message-ID: <alpine.DEB.1.10.0905071336430.22107@qirst.com>
Date:	Thu, 7 May 2009 13:40:01 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Alok Kataria <akataria@...are.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"alan@...rguk.ukuu.org.uk" <alan@...rguk.ukuu.org.uk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] x86: Reduce the default HZ value

On Thu, 7 May 2009, Peter Zijlstra wrote:

> On Thu, 2009-05-07 at 13:20 -0400, Christoph Lameter wrote:
> > On Thu, 7 May 2009, Peter Zijlstra wrote:
> >
> > > Another user is RCU, the grace period is tick driven, growing these
> > > ticks by a factor 50 or so might require some tinkering with forced
> > > grace periods when we notice our batch queues getting too long.
> >
> > One could also schedule RCU via hrtimers with a large fuzz period?
>
> No, that's not the point, the longer these period are, the more
> callbacks you can accumulate in a period. You need a cap on the callback
> list, we already have seen DoS scenarios in this space.

At some point the RCU must run and if the callback list gets too long the
callbacks must be processed. Thats unavoidable.


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