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

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Message-Id: <1241723854.11251.166.camel@twins>
Date:	Thu, 07 May 2009 21:17:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
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>
Subject: Re: [PATCH] x86: Reduce the default HZ value

On Thu, 2009-05-07 at 13:50 -0400, Christoph Lameter wrote:
> On Thu, 7 May 2009, Peter Zijlstra wrote:
> 
> > No it won't, you want fairly decent involuntary preemption rate to keep
> > the full service latency at a usable figure.
> >
> > The problem with scheduling a hrtimer along with tasks is that at high
> > context switch rates the timer will never fire but you do pay the
> > overhead of programming the hardware each time, something that can be
> > about as expensive as the whole context switch itself.
> 
> What are high context switch rates? 1000 HZ? Generally it seems that
> context switches are bad for cpu caches and thus to be avoided.


We're talking about in excess of 250k switches a second.
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