On Mon, Dec 05, 2005 at 06:29:02PM +0200, Avi Kivity wrote:
> Dave Jones wrote:
>
> >I can't think of a single valid reason why a program would want
> >to know the MHz rating of a CPU. Given that it's a) approximate,
> >b) subject to change due to power management, c) completely nonsensical
> >across CPU vendors, and d) only one of many variables regarding CPU
> >performance, any program that bases any decision on the values found
> >by parsing that field of /proc/cpuinfo is utterly broken beyond belief.
> >
> >
> Sometimes you need extremely low overhead time measurements, which need
> not be too accurate. One way to do this is to dump rdtsc measurements
> into some array, and later scale it using the cpu frequency.
>
> I've done exactly this. The processes were pinned to their processors,
> and there was no frequency scaling in effect. It worked very well.
That code will break as soon as it's run on a CPU that uses P-states.
You can't "just scale" the value, there are other factors at work.
Dave
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