linux-kernel - Re: [RFC patch 15/15] LTTng timestamp x86

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Message-Id: <200810211211.00332.bjorn.helgaas@hp.com>
Date:	Tue, 21 Oct 2008 12:10:58 -0600
From:	Bjorn Helgaas <bjorn.helgaas@...com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	john stultz <johnstul@...ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"Luck, Tony" <tony.luck@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC patch 15/15] LTTng timestamp x86

On Monday 20 October 2008 04:29:07 pm H. Peter Anvin wrote:
> Linus Torvalds wrote:
> > 
> > But it's not going to solve the "hey, I have 512 CPU's, they are all on 
> > different boards, and no, they are _not_ synchronized to one global 
> > clock!".
> > 
> > That's why I'd suggest making _purely_ local time, and then aiming for 
> > something NTP-like. But maybe there are better solutions out there.
> 
> At the same time, it would definitely be nice to encourage vendors of 
> large SMP systems to provide a common root crystal (frequency standard) 
> for a single SMP domain.  Preferrably a really good one, TCXO or better.

A single root crystal is nice for us software guys.  But it often
also turns into a single point of failure, which the hardware guys
are always trying to eliminate.  So I think multiple crystals are
inevitable for the really large machines.

Bjorn
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