Re: [-rt] time-related problems with CPU frequency scaling

[Wolfgang Hoffmann <woho@woho.de>]

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