linux-kernel - Re: [tip:timers/ntp] ntp: adjust SHIFT

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Date:	Tue, 2 Jun 2009 02:20:39 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Rik van Riel <riel@...hat.com>
Cc:	John Stultz <johnstul@...ibm.com>, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org,
	Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [tip:timers/ntp] ntp: adjust SHIFT_PLL to improve NTP
	convergence


* Rik van Riel <riel@...hat.com> wrote:

> John Stultz wrote:
>> On Tue, 2009-06-02 at 01:22 +0200, Ingo Molnar wrote:
>
>>> I might be missing something here - but Linux converging faster  
>>> seems like a genuinely good thing. What non-Linux problem could  
>>> there be? Linux's convergence is really Linux's private issue. 
>>
>> Yea. It does seem that way. Miroslav can likely expand on the 
>> issue to help clarify, but as I understand it, the example is if 
>> you have a number of systems that are peers in an NTP network. 
>> All of them are using the same userland NTP daemon. However, if 
>> the rate of change that corrections are applied is different in 
>> half of them, you will have problems getting all the systems to 
>> converge together.
>
> Would this not be true already, because the convergence of Linux 
> system suddenly became a lot slower in 2.6.19?
>
> Damned if we do, damned if we don't - except the new behaviour 
> introduced by your patches is nicer.

Not just that - but there's calibration noise during bootup that can 
cause randomly distributed recalibrations as well. So other hosts in 
a mixed environment will see inconsistencies anyway, after every 
bootup.

NTP is all about being able to be resilient against time noise and 
being able to sync up to a common time base ASAP.

	Ingo
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