linux-kernel - Re: nosmp/maxcpus=0 or 1 -> TSC unstable

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Message-ID: <20080116153137.GA18553@elte.hu>
Date:	Wed, 16 Jan 2008 16:31:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Pete Wyckoff <pw@....edu>
Cc:	dean gaudet <dean@...tic.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: nosmp/maxcpus=0 or 1 -> TSC unstable


* Pete Wyckoff <pw@....edu> wrote:

> > pretty sure this is the culprit is that num_possible_cpus() > 1, 
> > which would mean cpu_possible_map contains the second cpu... but i'm 
> > not quite sure what the right fix is... or perhaps this is all 
> > intended.
> 
> We've seen the same problem.  We use gettimeofday() for timing of 
> network-ish operations on the order of 10-50 us.  But not having the 
> TSC makes gettimeofday() itself very slow, on the order of 30 us.

30 usecs is too much - even with pmtimer it's typically below 5 usecs. 
Could you run this on your box:

  http://people.redhat.com/mingo/time-warp-test/time-warp-test.c

and send back what it reports? (run it for a few minutes)

	Ingo
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