Re: Dual core Athlons and unsynced TSCs

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 2006-01-13 at 11:32 -0800, thockin@hockin.org wrote:
> On Fri, Jan 13, 2006 at 10:56:13AM -0800, Sven-Thorsten Dietrich wrote:
> > On Fri, 2006-01-13 at 10:55 -0800, thockin@hockin.org wrote:
> > > unless we can re-sync the TSCs often enough that apps don't notice.
> > 
> > You'd have to quantify that somehow, in terms of the max drift rate
> > (ppm), and the max resolution available (< tsc frequency).  
> > 
> > Either that, or track an offset, and use one TSC as truth, and update
> > the correction factor for the other TSCs as often as needed, maybe?
> > 
> > This is kind of analogous to the "drift" NTP calculates against a
> > free-running oscillator. 
> > 
> > So you'd be pushing that functionality deeper into the OS-core.
> > 
> > Dave Mills had that "hardpps" stuff in there for a while, it might be a
> > starting point.
> > 
> > Just some thoughts for now... 
> 
> There's some chatter here about a solution involving a lazy sync of TSCs
> to the HPET (or other) whenever an app calls rdtsc after a potentially
> unsyncing event.
> 
> For example, 'hlt' will initiate C1 which may cause clock ramping (and TSC
> skew).  We can trap rdtsc after a hlt and re-sync the TSCs to some truly
> monotonic source, like HPET.
> 
> I don't have all the details, some problems remain, and the work is not
> quite done yet, but it looks promising.
> 
> Even if we eventually get synced TSCs, it's too little too late.
> basically, anything in-kernel that uses rdtsc is bound to break, and any
> app that uses rdtsc had better be using CPU affinity.

I know this might be somewhat of an overhead, but what about resyncing
on wakeup of the idle function?  Isn't that where they fall apart?

-- Steve