Subject: Re: fscked clock sources revisited

From: jamal <[email-masked]>
Date: Mon, 30 Jul 2007 22:14:36 -0400

On Mon, 2007-30-07 at 18:37 -0700, David Miller wrote:

> Relatively speaking, the acpi_pm numbers are at least consistent and
> about as much as so as jiffies.  Jiffies numbers are also possibly
> better, at least in part, because of the decreased accuracy and errors
> propagating.

I am not sure jiffies gives innacurate results. It gives the best
performance on the dual xeon i tested on, all the time. Very consistent.
On its accuracy: I was able to validate it With pktgen generating
traffic and some external hardware tool capturing the interpacket gaps
etc.

> tsc acts as expected, since every time your cpu changes power
> management state (which it is going to do dynamically) the TSC
> rates change and thus the accuracy goes into outer-space.

Robert was saying he had even more bizare results with opteron given the
NUMA nature.

> There really isn't much that can be done by any of this.  These issues
> exist because of hardware limitations, nobody bothered to build
> x86/x86_64 systems with a system wide TICK register that is both
> impervious to cpu frequence scaling and also cheap to access.
> 
> So the above is what we basically have to live with :-)

That is a bummer. I am going to test with hpet when i get the chance
and perhaps turn off all the other sources if nothing good comes out; i
need my numbers ;->

cheers,
jamal