linux-kernel - Re: constant_tsc and TSC unstable

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Message-ID: <474FB608.4030508@zytor.com>
Date:	Thu, 29 Nov 2007 23:04:40 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Paul Rolland (ポール・ロラン)" 
	<rol@...be.net>
CC:	Michael Tokarev <mjt@....msk.ru>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: constant_tsc and TSC unstable

Paul Rolland (ポール・ロラン) wrote:
> 
>> Note that once TSC is disabled (it's using "jiffies" as far
>> as I can see), ntpd constantly speeds up and slows down the
>> clock, it jumps +/- 0.5sec every several minutes or hours -
>> I guess that's when ntpd process gets moved from one core
>> to another for whatever reason.  And an interesting thing
>> is that with 64bits kernel this TSC problem does not occur
>> on this very machine.
> Hmmmm.... That could make it a problem related to kernel rather than CPU.
>  
>> Something similar is reported on AMD X2 64 machines as well --
>> can't check right now.
> If I recall correctly, issues with AMD X2 where related to TSC being
> independant for each core and not constant (speed depending of C state).
> But the reason I raise the issue is that the Core2 reports constant TSC,
> so there is (IMHO) no reason for that.
> 

Well, "constant" doesn't mean "synchronized", but it might very well be 
that the Core2 could really benefit from synchronizing the TSCs manually 
like we used to.

On the other hand, I notice that most of the TSC warp values are 
relatively close to 2^32, so this could be a specific bug.

	-hpa

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