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

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Message-ID: <4A034C13.8070402@zytor.com>
Date:	Thu, 07 May 2009 14:01:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Christoph Lameter <cl@...ux.com>,
	Alok Kataria <akataria@...are.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

Peter Zijlstra wrote:
> 
> 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.
> 
> Although, I guess we could amortize that by not re-programming the timer
> when the existing timer is within a reasonable period (say 1ms) of the
> requested on.
> 

That seems like a reasonable optimization since we're talking about 
multi-kHz context switch rates, here.

	-hpa

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