Re: [PATCH v6 03/10] KVM: selftests: Use flag CLOCK_MONOTONIC_RAW for timing

[wangyanan (Y)]

On 2021/4/19 16:22, David Laight wrote:
> From: wangyanan (Y)
> > Sent: 19 April 2021 07:40
> >
> > Hi Paolo,
> >
> > On 2021/4/17 21:23, Paolo Bonzini wrote:
> > > On 30/03/21 10:08, Yanan Wang wrote:
> > > > In addition to function of CLOCK_MONOTONIC, flag CLOCK_MONOTONIC_RAW can
> > > > also shield possiable impact of NTP, which can provide more robustness.
> > > >
> > > > Suggested-by: Vitaly Kuznetsov<vkuznets@xxxxxxxxxx>
> > > > Signed-off-by: Yanan Wang<wangyanan55@xxxxxxxxxx>
> > > > Reviewed-by: Ben Gardon<bgardon@xxxxxxxxxx>
> > > > Reviewed-by: Andrew Jones<drjones@xxxxxxxxxx>
> > > I'm not sure about this one, is the effect visible?
> > In practice, difference between results got with CLOCK_MONOTONIC and
> > CLOCK_MONOTONIC_RAW
> > actually is too little to be visible. But if just in theory,
> > CLOCK_MONOTONIC_RAW can ensure time results
> > of the compared tests are based on the same local oscillator frequency,
> > which is not subject to possible
> > NTP frequency adjustment. Change in this patch seems like a bit of
> > optimization.
> The real annoyance is when NTP is realigning the local clock.
> This typically happens after boot - but can take quite a few
> minutes (don't think it can quite get to an hour).
> (I think something similar is caused by leap seconds.)
>
> During this period CLOCK_MONOTONIC can run at a significantly
> different rate from 'real time'.
> This may not matter for timing self tests, but is significant
> for RTP audio.
>
> The problem there is that you want the NTP corrected time
> during 'normal running' because the small correction (for
> crystal error) is useful.
>
> But the kernel HR timers are only defined for CLOCK_MONOTONIC
> and the userspace requests for CLOCK_MONOTONIC_RAW are likely
> to be real system calls.
>
> What you really want is a clock whose frequency is adjusted
> by NTP but doesn't have the NTP offset adjuctments.
> In reality this ought to be CLOCK_MONOTONIC.
Hi David,

I see now, much thanks for the above explanation. :)
Still have a lot to learn about this part.

Thanks,
Yanan
> 	David
>
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