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SubjectRe: [-rt] time-related problems with CPU frequency scaling
Date
On Tuesday 18 April 2006 00:35, Lee Revell wrote:
> On Sun, 2006-04-16 at 20:41 +0200, Wolfgang Hoffmann wrote:
> > - if CPU frequency is low when jackd is started, it complains:
> > "delay of 2915.000 usecs exceeds estimated spare
> > time of 2847.000; restart ..."
> > as soon as frequency is scaled up. Seems that jackd gets confused by
> > some influence of CPU frequency on timekeeping? No problems as long as
> > CPU frequency isn't scaled up, though.
>
> JACK still uses the TSC for timing and thus is incompatible with CPU
> frequency scaling. You must use the -clockfix branch from CVS.

Ok, so that's a userspace issue? Thanks for the pointer, I'll try the CVS
branch.

> > - values in /proc/sys/kernel/preempt_max_latency scales inverse to
> > CPU frequency, i.e. 24us with CPU @ 800MHz and 12us with CPU @ 1,6GHz
>
> This is normal - code that takes 12us to run at 1.6 GHz will take 24us
> at 800MHz. TANSTAAFL ;-)

So I misunderstood preempt_max_latency. I thought it to be absolute time, but
it actually is codepath cycles, translated to microseconds using the current
CPU frequency. Thanks for clarifying.

Wolfgang
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