Messages in this thread | | | From | Wolfgang Hoffmann <> | Subject | Re: [-rt] time-related problems with CPU frequency scaling | Date | Tue, 18 Apr 2006 08:11:13 +0200 |
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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.
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