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

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Date:   Fri, 10 Mar 2023 02:35:21 +0000
From:   "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>
To:     Akira Yokosawa <akiyks@...il.com>,
        "paulmck@...nel.org" <paulmck@...nel.org>
CC:     "frederic@...nel.org" <frederic@...nel.org>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "urezki@...il.com" <urezki@...il.com>
Subject: RE: [PATCH v3] rcu: Add a minimum time for marking boot as completed

> From: Akira Yokosawa <akiyks@...il.com>
> Sent: Friday, March 10, 2023 8:12 AM
> To: paulmck@...nel.org; Zhuo, Qiuxu <qiuxu.zhuo@...el.com>
> Cc: 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; Akira Yokosawa
> <akiyks@...il.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@...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 Yokosawa for sharing the histogram to provide an 
intuitive view of these data points and your analysis. ;-)

-Qiuxu