Miroslav, On Fri, 2 Nov 2018, Miroslav Lichvar wrote: > On Thu, Nov 01, 2018 at 06:41:00PM +0100, Thomas Gleixner wrote: > > On Wed, 24 Oct 2018, Miroslav Lichvar wrote: > > > The error is too large to be corrected by stepping on clock updates. > > > For a typical TSC frequency we have multiplier in the range of few > > > millions, so that's a frequency error of up to few hundred ppb. In the > > > old days when the clock was updated 1000 times per second that would > > > be hidden in the resolution of the clock, but now with tickless > > > kernels those steps would be very noticeable. > > > That only happens when the system was completely idle for a second and in > > that case it's a non issue because the clock is updated before it's > > used. So nothing will be able to observe the time jumping forward by a few > > or even a few hundreds of nanoseconds. > > That's great news (to me). I think we should do the same with the > mono/real clock. A periodic 4ns step would be better than a slew > correcting tens or hundreds of nanoseconds. This would be a > significant improvement in accuracy on idle systems, in theory > identical to running with nohz=off. > > Maybe I am missing some important detail, but I think we can just drop > the +1 mult adjustment and step on each update by the (truncated) > amount that has accumulated in the NTP error register. With the > changes that have been made earlier this year the clock should never > be ahead, so the step would always be forward. That sounds reasonable. > > For the regular case, where CPUs are > > busy and the update happens 100/250/1000 times per second the jump forward > > will not be noticable at all. > > I think a 4ns jump at 100 Hz might be noticeable with a good reference > clock and large number of measurements, but so would be the current +1 > mult adjustment. Indeed. Thanks, tglx
