linux-kernel - Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed

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Date:   Thu, 9 Mar 2023 17:47:54 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Akira Yokosawa <akiyks@...il.com>
Cc:     qiuxu.zhuo@...el.com, frederic@...nel.org, jiangshanlai@...il.com,
        joel@...lfernandes.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org, urezki@...il.com
Subject: Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed

On Fri, Mar 10, 2023 at 09:11:54AM +0900, Akira Yokosawa wrote:
> 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@...nel.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, Akira!  Definitely an abnormal distribution!  ;-)

							Thanx, Paul