Re: [PATCH] CPU frequency display in /proc/cpuinfo

[Dave Jones]

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

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/