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Re: Why is this happening?

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On Mon, 30 Mar 2026 09:25:39 +0200
Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:

> On 2026-03-30 01:41, Mike Terry wrote:
> > On 29/03/2026 13:30, Janis Papanagnou wrote:  
> >> On 2026-03-28 21:58, Josef Möllers wrote:  
> >>> On 27.03.26 17:02, Scott Lurndal wrote:  
> >>>> =?UTF-8?Q?Josef_M=C3=B6llers?= <josef@invalid.invalid> writes:  
> >>>>> On 27.03.26 05:46, Janis Papanagnou wrote:  
> >>>>>> Subject: Why is this happening?
> >>>>>>
> >>>>>> I think because you are doing wrong calculations here:
> >>>>>>
> >>>>>>   > return ((stop.tv_sec-started.tv_sec) + (stop.tv_nsec-
> >>>>>> started.tv_nsec)) / B;
> >>>>>>
> >>>>>> You should not subtract the components s/ns separately when
> >>>>>> scaling. Instead build the float stop number and started
> >>>>>> number separately and then subtract.  
> >>>>>
> >>>>> I usually subtract the nsec parts and if the result is
> >>>>> negative, adjust
> >>>>> the sec and the nsec parts accordingly:  
> >>
> >> That's fine if you operate on the integer parts separately. (I'm
> >> sometimes doing a similar thing if I'm working with the integral
> >> components of a calendar date, for example.)
> >>
> >> Generally I prefer to use types that fit the element I'm operating
> >> on. That's why I prefer to express  dT = stop_time - start_time
> >> in the case above as
> >>    (stop_s + F * stop_ns) - (start_s + F * start_ns)   with F=1e-9
> >> to easily see the application entities and logic.
> >>
> >> I think that's the clearest expression, and it's an expression
> >> without conditional. (A habit from the early days of programming
> >> where conditionals were suboptimal. Nowadays it's meaningless,
> >> and huge amounts of time wasted at other places; using suboptimal
> >> algorithms, or over-complicated processes, or just bloat.) The
> >> expressions in your variant appear to me unnecessary confusing
> >> with using '1000000000UL', for example. - As said, basically a
> >> matter of personal preference, despite the rationale of clarity,
> >> which is also mostly subjective.
> >>  
> > We have a calculation here, which ignoring precision issues is of
> > the form
> > 
> >     (A1  +  A2)  -  (B1  +  B2)
> > 
> > The magnitudes are such that A1,B1 are some order of magnitude
> > greater than A2,B2, but the differences A1-B1, A2-B2 are of broadly
> > similar magnitude (in the scenario we're considering).  If we were
> > using a single integral type for everything, or some kind of
> > infinite precision real numbers, the order of calculation would not
> > matter, but floating point is not like that - the order of
> > calculation can give different results, due to the finite precision
> > limits.
> > 
> > In such a situation, it's surely good practice to subtract the 
> > components of similar magnitude first, then add the results, as
> > this avoids unnecessarily dropping the precision of the final
> > result.  That seems more important than how subjectively clear the
> > expression might look to someone. :)
> > 
> > So I'd go with the original (A1-B1) + (A2-B2) expression, even if 
> > someone does some calculation to show that on a particular
> > architecture, with the actual float/time types being used, the
> > "logical/clearest expression" ordering ((A1+A2) - (B1+B2)) is
> > /sufficiently/ precise for purposes.  
> 
> Mike, I see where you're coming from; from numerical mathematics. In
> cases where you have to accumulate many values of different magnitude
> what you describe is necessary to not lose the values of minor value
> in the final sum, since a large amount of such values may contribute.
> Here we were calculating run-times. A "problem" arises if you cannot
> represent huge seconds values combined with high resolution nano-sec
> values in one, say, double variable. If one thinks that, say, with
> runtime of minutes or hours the least nanoseconds resolution is (for
> your case) of interest you may do that. (And mind that we're still
> not accumulating many such values!) But usually an IEEE double float
> with 64 bit and ~16 decimal figures should suffice for that purpose.

No, it isn't.
IEEE binary64 a.k.a. C double is capable to represent exact nanoseconds
only up to 104 days. So, for UNIX epoch, it had ran out of nanoseconds
precision a year or two before KT had first working system.
By now, Unix time presented in 'double' would have delta=256nsec.
With Windows FILETIME epoch (January 1, 1601) it would be 8 times worse.

OTOH, 64-bit signed integer is sufficient for nsec resolution with Unix
epoch, but insufficient (by one bit) with Windows FILETIME epoch.


> YMMV. 

No, it is not a matter of mileage. There is a correct way (Mike's and
DFS's and of just about everybody else's who posted in this thread) and
incorrect way - yours.

> For me, having seen a lot of good and bad code, clarity of code
> is an essential software property so I'd not unnecessarily muddy the
> code for a requirement we actually don't seem to have (in this case).
> 
> Janis
> 

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Thread

Why is this happening? DFS <nospam@dfs.com> - 2026-03-27 00:12 -0400
  Re: Why is this happening? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-03-27 05:46 +0100
    Re: Why is this happening? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-03-27 06:01 +0100
      Re: Why is this happening? DFS <nospam@dfs.com> - 2026-03-27 02:31 -0400
    Re: Why is this happening? Josef Möllers <josef@invalid.invalid> - 2026-03-27 10:15 +0100
      Re: Why is this happening? scott@slp53.sl.home (Scott Lurndal) - 2026-03-27 16:02 +0000
        Re: Why is this happening? Josef Möllers <josef@invalid.invalid> - 2026-03-28 21:58 +0100
          Re: Why is this happening? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-03-29 14:30 +0200
            Re: Why is this happening? scott@slp53.sl.home (Scott Lurndal) - 2026-03-29 18:56 +0000
              Re: Why is this happening? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-03-30 02:00 +0200
            Re: Why is this happening? Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2026-03-30 00:41 +0100
              Re: Why is this happening? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-03-30 09:25 +0200
                Re: Why is this happening? Michael S <already5chosen@yahoo.com> - 2026-03-30 11:23 +0300
                Re: Why is this happening? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-03-30 11:16 +0200
                Re: Why is this happening? Michael S <already5chosen@yahoo.com> - 2026-03-30 13:06 +0300
      Re: Why is this happening? Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-03-28 01:46 +0000
        Re: Why is this happening? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-27 20:41 -0700
          Re: Why is this happening? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-03-28 05:17 +0100
            Re: Why is this happening? David Brown <david.brown@hesbynett.no> - 2026-03-28 10:09 +0100
              Re: Why is this happening? Bart <bc@freeuk.com> - 2026-03-28 11:33 +0000
                Re: Why is this happening? David Brown <david.brown@hesbynett.no> - 2026-03-28 16:35 +0100
                Re: Why is this happening? Richard Harnden <richard.nospam@gmail.invalid> - 2026-03-28 16:56 +0000
                Re: Why is this happening? Richard Harnden <richard.nospam@gmail.invalid> - 2026-03-28 16:58 +0000
                Re: Why is this happening? James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-03-28 19:24 -0400
                Re: Why is this happening? David Brown <david.brown@hesbynett.no> - 2026-03-29 11:33 +0200
                Re: Why is this happening? Richard Harnden <richard.nospam@gmail.invalid> - 2026-03-29 16:30 +0100
                Re: Why is this happening? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-29 10:37 -0700
                Re: Why is this happening? David Brown <david.brown@hesbynett.no> - 2026-03-31 16:20 +0200
                Re: Why is this happening? Richard Harnden <richard.nospam@gmail.invalid> - 2026-03-31 17:19 +0100
                Re: Why is this happening? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-03-29 13:19 -0700
                Re: Why is this happening? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-03-29 13:26 -0700
                Re: Why is this happening? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-29 14:14 -0700
                Re: Why is this happening? James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-03-29 20:47 -0400
                Re: Why is this happening? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-29 13:57 -0700
                Re: Why is this happening? David Brown <david.brown@hesbynett.no> - 2026-03-31 16:26 +0200
          Re: Why is this happening? scott@slp53.sl.home (Scott Lurndal) - 2026-03-28 15:14 +0000
            Re: Why is this happening? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-28 10:18 -0700
              Re: Why is this happening? scott@slp53.sl.home (Scott Lurndal) - 2026-03-29 18:50 +0000
            Re: Why is this happening? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-28 19:12 -0700
              Re: Why is this happening? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-28 22:31 -0700
                Re: Why is this happening? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-04-06 12:11 -0700
                Re: Why is this happening? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-04-06 14:20 -0700
                Re: Why is this happening? Michael S <already5chosen@yahoo.com> - 2026-04-07 02:05 +0300
                Re: Why is this happening? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-04-06 17:42 -0700
              Re: Why is this happening? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-29 14:43 -0700
            Re: Why is this happening? Michael S <already5chosen@yahoo.com> - 2026-03-29 11:50 +0300
              Re: Why is this happening? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-29 13:54 -0700
                Re: Why is this happening? Michael S <already5chosen@yahoo.com> - 2026-03-30 02:05 +0300
                Re: Why is this happening? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-29 16:22 -0700
                Re: Why is this happening? Michael S <already5chosen@yahoo.com> - 2026-03-30 10:57 +0300
                Re: Why is this happening? James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-03-30 20:37 -0400
                Re: Why is this happening? Michael S <already5chosen@yahoo.com> - 2026-03-31 11:48 +0300
                Re: Why is this happening? James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-03-31 10:51 -0400
                Re: Why is this happening? candycanearter07 <candycanearter07@candycanearter07.nomail.afraid> - 2026-03-31 19:00 +0000
        Re: Why is this happening? Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-04-03 02:41 +0100

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