linux-kernel - Re: 2.6.18 tsc clocksource + ntp = excessive drift; acpi

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Date:	Tue, 28 Nov 2006 23:29:49 -0200
From:	Alexandre Pereira Nunes <alexandre.nunes@...il.com>
To:	john stultz <johnstul@...ibm.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: 2.6.18 tsc clocksource + ntp = excessive drift; acpi_pm does
 fine.

john stultz escreveu:

>On Tue, 2006-11-28 at 21:46 -0200, Alexandre Pereira Nunes wrote:
>  
>
>>Hi,
>>
>>with default boot I got tsc clocksource selected on an debian's
>>2.6.18-3-k7 SMP build (but UP machine). ntp keeps bothering me with this
>>message:
>>frequency error 512 PPM exceeds tolerance 500 PPM
>>    
>>
>
>Hmmm. Could you send me your dmesg? Also what frequency is your cpu?
>
>  
>
Sure, attached!
You'll notice an "acpi_pm installed" or something at the end, that was 
at the time I typed the echo acpi_pm >/sys/whatever.

My cpu is an athlon xp 2600+, I attached a copy of /proc/cpuinfo for 
convenience.

>Also does booting w/ "noapic" change the behavior?
>  
>
I'll test it and let you know. I also read (but didn't try) about some 
"notsc" option, I assume that's not a good one to try, right?

> [cut]
>
>>If I remove ntp's drift file, then do a: echo acpi_pm
>> >/sys/devices/system/clocksource/clocksource0/available_clocksource ;
>>    
>>
>
>I think you mean "current_clocksource" there...
>
>  
>
Ooops. Let's just pretend no one else saw that! :-)

>[cut]
>Yea, its likely the generic timekeeping changes for i386. Previously
>(pre-2.6.18) it probably defaulted to the acpi pm timer and was fine.
>The new code is a bit more aggressive in trying to use the TSC.
>  
>
Just out of curiousity: what about this acpi_pm stuff ... Reading from 
tsc is probably cheaper than any other "accurate" clock source, but how 
bad (or good) is acpi_pm?

>As a short term workaround, you can put "clocksource=acpi_pm" on your
>grub line and that will force the clocksource at boot.
>  
>

Yeah, I googled around and had put that on grub's config, but didn't 
reboot. I'll swap that with noapic and reboot, by tomorrow I should have 
some news.

- Alexandre


View attachment "dmesg.txt" of type "text/plain" (16554 bytes)

View attachment "cpuinfo.txt" of type "text/plain" (419 bytes)