linux-kernel - Re: One-shot high-resolution POSIX timer periodically late

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Message-ID: <20070206123716.GA8481@elte.hu>
Date:	Tue, 6 Feb 2007 13:37:16 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	John <shill@...e.fr>
Cc:	linux-kernel@...r.kernel.org, tglx@...esys.com,
	johnstul@...ibm.com, akpm@...l.org
Subject: Re: One-shot high-resolution POSIX timer periodically late


* John <shill@...e.fr> wrote:

> > 2.6.18-rt7 is pretty old, do you see the problem in 2.6.20-rc6-rt5 
> > too?
> 
> Ingo, Thomas,
> 
> I cannot reproduce in 2.6.20-rc6-rt6, but it is not strictly an 
> apples-to-apples comparison because I get a tsc clocksource in 
> 2.6.20-rc6-rt6, and an acpi_pm clocksource in 2.6.18.6-rt7.

that suggests that the TSC was not deemed reliable by the newer kernel - 
which almost always means it's not realiable. I think the problems you 
were seeing are consistent with TSC being non-monotonic - so the fix is 
to not use the TSC - and the newer kernel does this automatically.

	Ingo
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