Re: Question about ktime_get_mono_fast_ns() non-monotonic behavior

[John Stultz]

On Thu, Oct 13, 2022 at 6:29 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Thu, Oct 13, 2022 at 5:03 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> >
> > On Wed, Oct 12, 2022 at 8:07 PM John Stultz <jstultz@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Oct 12, 2022 at 8:02 PM John Stultz <jstultz@xxxxxxxxxx> wrote:
> > > > On Mon, Sep 26, 2022 at 2:18 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> > > > So I think it reasonable to say its bounded by approximately  2 *
> > > > NSEC_PER_SEC/HZ +/- 11%.
> > >
> > > Sorry, this should be 2*NSEC_PER_SEC/HZ * 0.11
> >
> > Thanks so much for the detailed response :)
> >
> > IIUC this error bound is in ns. So on a 2 GHz cpu the bound is 0.11 ns
> > (essentially 0)? I feel like I miscalculated, this error bound is too
> > good to be true.
>
> Never mind, I thought HZ is the cpu speed for some reason. It's the
> number of jiffies per second, right?

Correct.

> So if HZ is 1000, the error bound is actually ~2 ms, which is very
> large considering that the unit is ns.

Uh, for HZ=1000, I think it's closer to 220us, but yes, for HZ=100 2.2ms.

thanks
-john