Re: Intel P6 vs P7 system call performance

From: Pavel Machek (pavel@ucw.cz)
Date: Thu Dec 19 2002 - 17:26:14 EST


Hi!

> >>>>don't many of the multi-CPU problems with tsc go away because you've got a
> >>>>per-cpu physical page for the vsyscall?
> >>>>
> >>>>i.e. per-cpu tsc epoch and scaling can be set on that page.
> >>>
> >>>Problem is that cpu's can randomly drift +/- 100 clocks or so... Not
> >>>nice at all.
> >>>
> >>
> >>ญญ?100 clocks is what... ?50 ns these days? You can't get that kind of
> >>accuracy for anything outside the CPU core anyway...
> >
> > 50ns is bad enough when it makes your time go backwards.
> >
>
> Backwards?? Clock spreading should make the rate change, but it should
> never decrement.

User on cpu1 reads time, communicates it to cpu2, but cpu2 is drifted
-50ns, so it reads time "before" time reported cpu1. And gets confused.

                                                                Pavel

-- 
Casualities in World Trade Center: ~3k dead inside the building,
cryptography in U.S.A. and free speech in Czech Republic.
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