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

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Date:	Tue, 2 Jun 2009 09:47:36 -0800 (AKDT)
From:	"Mr. James W. Laferriere" <babydr@...y-dragons.com>
To:	Ray Lee <madrabbit@...il.com>
cc:	John Stultz <johnstul@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	mingo@...hat.com, hpa@...or.com, riel@...hat.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

 	Hello All ,

On Mon, 1 Jun 2009, Ray Lee wrote:
> On Mon, Jun 1, 2009 at 4:58 PM, John Stultz <johnstul@...ibm.com> wrote:
>> On Tue, 2009-06-02 at 01:22 +0200, Ingo Molnar wrote:
>>> * John Stultz <johnstul@...ibm.com> wrote:
>>>> On Wed, 2009-05-06 at 09:46 +0000, tip-bot for john stultz wrote:
>>>>> ntp: adjust SHIFT_PLL to improve NTP convergence
>>>>>
>>>>> The conversion to the ntpv4 reference model
>>>>> f19923937321244e7dc334767eb4b67e0e3d5c74 ("ntp: convert to the NTP4
>>>>> reference model") in 2.6.19 added nanosecond resolution the adjtimex
>>>>> interface, but also changed the "stiffness" of the frequency adjustments,
>>>>> causing NTP convergence time to greatly increase.
>>>>>
>>>>> SHIFT_PLL, which reduces the stiffness of the freq adjustments, was
>>>>> designed to be inversely linked to HZ, and the reference value of 4 was
>>>>> designed for Unix systems using HZ=100.  However Linux's clock steering
>>>>> code mostly independent of HZ.
>>>>>
>>>>> So this patch reduces the SHIFT_PLL value from 4 to 2, which causes NTPd
>>>>> behavior to match kernels prior to 2.6.19, greatly reducing convergence
>>>>> times, and improving close synchronization through environmental thermal
>>>>> changes.
>>>>>
>>>>>
>>>>> [ Impact: tweak NTP algorithm for faster convergence ]
>>>>
>>>>     So I've been speaking with Miroslav (cc'ed) who maintains
>>>> the RH ntpd packages, and he's concerned that this patch takes us
>>>> out of NTP's expected behavior, which may cause problems when
>>>> dealing with non-linux systems using NTP.
>>>
>>> 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.
>
> Your point is clear, however -- reasonably speaking --  how many
> instances will there be out there of networks of peers partially
> upgraded versus lone systems slowly or never converging off of
> masters?
 	A site with three or four differant system types ,  ie: sparc running 
sloaris ,  pc running openbsd ,  Dec(hp) running VMS ,  Dec(hp) Alpha running 
Linux , ...
 	This moving the Hardware Arch & OS differencews is sometimes done to 
limit hacks & software glicthes by NOT using the same hardware arch or OS .

> By my naive understanding, the latter would strongly outnumber the former.
 	You'd be VERY unhappily suprised .

 		Hth ,  JimL
-- 
+------------------------------------------------------------------+
| James   W.   Laferriere | System    Techniques | Give me VMS     |
| Network&System Engineer | 2133    McCullam Ave |  Give me Linux  |
| babydr@...y-dragons.com | Fairbanks, AK. 99701 |   only  on  AXP |
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