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Message-ID: <20090602002039.GA16410@elte.hu>
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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