RE: [PATCH v3] rcu: Add a minimum time for marking boot as completed

Zhuo, Qiuxu
2023-03-10T02:35:37Z

> From: Akira Yokosawa <akiyks@gmail.com>
> Sent: Friday, March 10, 2023 8:12 AM
> To: paulmck@kernel.org; Zhuo, Qiuxu <qiuxu.zhuo@intel.com>
> Cc: frederic@kernel.org; jiangshanlai@gmail.com; joel@joelfernandes.org;
> linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org;
> rcu@vger.kernel.org; urezki@gmail.com; Akira Yokosawa
> <akiyks@gmail.com>
> Subject: Re: [PATCH v3] rcu: Add a minimum time for marking boot as
> completed
>
> Hi,
>
> Let me chime in this interesting thread.
>
> On Thu, 9 Mar 2023 13:53:39 -0800, Paul E. McKenney wrote:
> > On Thu, Mar 09, 2023 at 03:17:09PM +0000, Zhuo, Qiuxu wrote:
> >> > From: Paul E. McKenney <paulmck@kernel.org> [...]
> >> > >
> >> > > a's standard deviation is ~0.4.
> >> > > b's standard deviation is ~0.5.
> >> > >
> >> > > a's average 9.0 is at the upbound of the standard deviation of b's [8.0,
> 9].
> >> > > So, the measurements should be statistically significant to some
> degree.
> >> >
> >> > That single standard deviation means that you have 68% confidence
> >> > that the difference is real. This is not far above the 50% leval of random
> noise.
> >> > 95% is the lowest level that is normally considered to be
> >> > statistically significant.
> >>
> >> 95% means there is no overlap between two standard deviations of a
> >> and two standard deviations of b.
> >>
> >> This relies on either much less noise during testing or a big enough
> >> difference between a and b.
>
> Appended is a histogram comparing 2 data sets.
>
> As you see, the one with v2 patch is far from normal distribution.
> I think there is at least two peaks.
> The one at the right around 9.7 seems not affected by the patch.
> In such a case, average and standard deviation of all the data don't tell much.
>
> It is hard to say anything for sure with such small set of samples.
> And the shape of the plot is likely to be highly dependent on machine setups.
>
> Hope this helps.

Thank you Yokosawa for sharing the histogram to provide an
intuitive view of these data points and your analysis. ;-)

-Qiuxu