Home Original page

[Python-ideas] Why not picoseconds?

Victor Stinner victor.stinner at gmail.com
Mon Oct 16 09:10:38 EDT 2017 
2017-10-16 9:46 GMT+02:00 Stephan Houben <stephanh42 at gmail.com>:
> Hi all,
>
> I realize this is a bit of a pet peeve of me, since
> in my day job I sometimes get people complaining that
> numerical data is "off" in the sixteenth significant digit
> (i.e. it was stored as a double).
> (...)

Oh. As usual, I suck at explaining the rationale. I'm sorry about that.

The problem is not to know the number of nanoseconds since 1970. The
problem is that you lose precision even on short durations, say less
than 1 minute. The precision loss depends on the reference of the
clock, which can be the UNIX epoch, a reference based on the computer
boot time, a reference based on the process startup time, etc.

Let's say that your web server runs since 105 days, now you want to
measure how much time it took to render a HTML template and this time
is smaller than 1 ms. In such case, the benchmark lose precision just
because of the float type, not because of the clock resolution.

That's one use case.

Another use case is when you have two applications storing time. A
program A writes a timestamp, another program B compares the timestamp
to the current time. To explain the issue, let's say that the format
used to store the timestamp and clock used by program A have a
resolution of 1 nanosecond, whereas the clock used by program B has a
resolution of 1 second.  In that case, there is a window of 1 second
where the time is seen as "created in the future". For example, the
GNU tar program emits a warning in that case ("file created in the
future", or something like that).

More generally, if you store time with a resolution N and your clock
has resolution P, it's better to have N >= P to prevent bad surprises.
More and more databases and filesystems support storing time with
nanosecond resolution.

Victor
More information about the Python-ideas mailing list