linux-kernel - Re: [RFC][PATCH] Introduce CLOCK_REALTIME

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Message-ID: <20090722213942.GA4337@josh-work.beaverton.ibm.com>
Date:	Wed, 22 Jul 2009 14:39:46 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	john stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...x.de>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <ak@...e.de>, nikolag@...ibm.com,
	Darren Hart <dvhltc@...ibm.com>
Subject: Re: [RFC][PATCH] Introduce CLOCK_REALTIME_COARSE

On Mon, Jul 20, 2009 at 03:49:14PM +0200, Peter Zijlstra wrote:
> On Mon, 2009-07-20 at 06:33 -0700, Arjan van de Ven wrote:
> > > Maybe power64, sparc64 and s390x qualify, but certainly nothing on x86
> > > does.
> > 
> > the x86 on my desk disagrees.
> 
> >From what I know even nehalem doesn't have fully synced tscs when your
> machine is large enough, and the timers are still a tad expensive.

All single-socket Nehalem boards have synced TSCs, and some multi-socket
Nehalem boards have synced TSCs, notably Intel's.  I don't know if you
can safely rely on the latter, though.

> Maybe your desktop is next-gen? 't would be nice to finally have an x86
> that has usable clock and timer hardware.

x86 CPUs with the ARAT feature ("always running APIC timer") will let
you use the local APIC even in deep C-states.  I don't know about the
overhead of the local APIC.

- Josh Triplett
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