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SubjectRe: do_gettimeofday vs. rdtsc in the scheduler
On Tue, Sep 17, 2002 at 03:02:04PM -0700, James Cleverdon wrote:
> On Tuesday 17 September 2002 02:18 pm, David S. Miller wrote:
> > From: Alan Cox <alan@lxorguk.ukuu.org.uk>
> > Date: 17 Sep 2002 22:28:12 +0100
> >
> > A bus clock - but things like the x440 have more than one bus clock. Its
> > NUMA. Also the bus clock and rdtsc clock are different - rdtsc is
> > dependant on the multiplier. Shove a celeron 300 and a celeron 450 in a
> > BP6 board with tsc on and enjoy
> >
> > That's mostly my point.
> >
> > If the bus clocks differ, then great create some system wide crystal
> > oscillator. That's a detail, the important bit is that you don't need
> > to go out to the system bus to read the tick value, it must be cpu
> > local to be effective and without serious performance impact.
> > -
>
> It's more than just a detail. Sequent's last NUMA system (_not_ the NUMA-Q;
> never released) did exactly what you suggest. The midplane card generated
> the bus clock for all quad modules. We had requested this feature because it
> was such a pain dealing with clock drift between nodes in the OS.
>
> The HW guys were able to give us synchronized bus clocks on a 16-way box, but
> warned us that it would not be practical on the 256-way. Too much clock skew
> at those speeds, or something like that. I suppose you could trade off
> interconnect rate for clock sync, but then performance would suffer.
>
> 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.
>
> Interestingly, Intel's IA64 manual does not guarantee that the CPU clock (and
> thus its TSC register) has anything to do with the bus clock rate. Maybe
> they want to dabble with asynchronous logic or multiple clock domains in
> future CPUs.

The point here is: You don't need a synchronized bus clock. You don't
need synchronized CPU clocks. You need a synchronized system-wide clock
that doesn't drive any bus or CPU, just a simple counter in every CPU
that you can read from inside the CPU. You can pull that pretty far and
to many CPUs. That's what I understand Sun does.

--
Vojtech Pavlik
SuSE Labs
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