[ntp:questions] Re: 'summary.in' and 'plot_summary.in

Ulrich Windl Ulrich.Windl at RZ.Uni-Regensburg.DE
Wed Jan 26 14:46:06 UTC 2005

"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

> Nigel Smith wrote:
> >I am looking for ways to analyze and graph the statistics generated by
> >NTP and came across the two perl files 'summary.in' and
> >'plot_summary.in' in the scripts directory of ntp-4.2.0.tar.gz
> >
> >Is any one else using these scripts? What is your experience with them?

I did convert the awk (I think it was) versions to Perl and made them Y2K
compliant (I thought).

> >
> >When I try analyzing my statistics, with 'summary.in', I am getting a
> >few error lines like this:
> >Can't take sqrt of -384.314 at /root/summary.pl line 326.
> >Can't take sqrt of -427.51 at /root/summary.pl line 326.
> >Can't take sqrt of -1.63582e+08 at /root/summary.pl line 326.
> >
> >How worried should I be by these errors? What could be causing them?
> >Has any one got a fix for the code?
> >
> >Can anyone recommend any other good ways to analyze and graph the
> >statistics file, both on a daily basis and also more long term
> >analysis?
> >
> >Thanks
> >Nigel Smith
> >
> >
> I just had a *very* quick look at that code.  It looks as if the author fell
> into a  relatively well known trap.
> Variance is defined as "the square root of the sum of the squares of the
> sample deviations from the mean divided by the number of samples.  The safe
> way to calculate this requires that you first find the mean of the samples and
> then calculate the sum of the squares of the deviations from the mean.  Many
> programmers use a shortcut that is mathematically equivalent but is not
> "computationally equivalent" when using floating point numbers.
> That shortcut looks like this (in pseudo code)
> a=0
> for i = 1 to N
> {
> s +=x**2
> t+=x
> }
> sigma = s/N  - (t/N)**2
>  Computations using a limited number of significant figures can result in a
> mathematically impossible result of a negative variance.   When you then try
> to calculate the standard deviation by finding the square root of the
> variance, the program blows up in your face.
> It looks as if the code does something similar.   The only sure fix is to
> rewrite the code to do the calculation in a way that is both mathematically
> and computationally correct!  If perl supports "double precision" the
> additional significant figures might alleviate the problem somewhat but
> probably not eliminate it completely.

Perl does double unless instructed otherwise. Try "perl -e 'print sqrt(2)'"; I
get "1.4142135623731". Is that double enough?

I doubt to believe I'm the only Perl programmer out there!


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