Re: do_gettimeofday vs. rdtsc in the scheduler

From: James Cleverdon (
Date: Tue Sep 17 2002 - 17:55:52 EST

On Tuesday 17 September 2002 03:38 pm, David S. Miller wrote:
> From: Andi Kleen <>
> Date: Wed, 18 Sep 2002 00:44:42 +0200
> > I don't know how Sun and SGI manage with their larger systems. Either
> > they don't do clock sync, or they may have to make expensive
> > tradeoffs.
> I guess you could always run NTP between the different CPUs ;) ;)
> :-)
> More seriously, you don't need to have the cpu tick registers sync'd,
> it is the rate that matters.
> Once booted, you can sync these system tick registers with a pretty
> straight forward algorithm in the kernel. Bonus points if you can
> figure out how to cancel out the cost of moving the system tick sample
> cachelines between master and slave in your algorithm :-)

Been there. Done that. Had the product canceled. ;^)

The initial sync was easy, even with variable latencies on cache lines. A
much simplified NTP-ish algorithm works fine. The painful thing was bus
clock drift and programs that foolishly relied on the TSC being the same
between CPUs and between nodes.

James Cleverdon
IBM xSeries Linux Solutions
{jamesclv(Unix, preferred), cleverdj(Notes)} at us dot ibm dot com

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