linux-kernel - Re: [discuss] Re: [PATCH] Allow all Opteron processors to change pstate at same time

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Date:	Wed, 26 Jul 2006 20:53:40 +0200
From:	Andi Kleen <ak@...e.de>
To:	discuss@...-64.org
Cc:	"Langsdorf, Mark" <mark.langsdorf@....com>,
	"Gulam, Nagib" <nagib.gulam@....com>, linux-kernel@...r.kernel.org,
	cpufreq@...ts.linux.org.uk
Subject: Re: [discuss] Re: [PATCH] Allow all Opteron processors to change pstate at same time

> In contrast, the same machine running with TSC and standard
> PN! sees massive drift, upwards of an hour, within an hour.

Do you see the same drift when you lock date on a single CPU
with taskset?

> If the TSCnow! patch reduces measured drift down to a second 
> a week, would you consider that acceptable?

No because even one second a week will break timing badly
over time. 

I believe the only good solution unless hardware helps would
be to use per CPU TSC offsets as discussed earlier. Even that
is a bit risky because there can be still very small drifts,
but they should be limited by a clock tick error max and
might work out.

-Andi

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