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[Python-Dev] cpython: Issue #10278: Add an optional strict argument to time.steady(), False by default

Matt Joiner anacrolix at gmail.com
Tue Mar 20 08:33:51 CET 2012 
I believe we should make a monotonic_time method that assures monotonicity
and be done with it. Forward steadiness can not be guaranteed. No
parameters.
On Mar 20, 2012 2:56 PM, "Steven D&apos;Aprano" <steve at pearwood.info> wrote:

> On Mon, Mar 19, 2012 at 01:35:49PM +0100, Victor Stinner wrote:
>
> > Said differently: time.steady(strict=True) is always monotonic (*),
> > whereas time.steady() may or may not be monotonic, depending on what
> > is avaiable.
> >
> > time.steady() is a best-effort steady clock.
> >
> > (*) time.steady(strict=True) relies on the OS monotonic clock. If the
> > OS provides a "not really monotonic" clock, Python cannot do better.
>
> I don't think that is true. Surely Python can guarantee that the clock
> will never go backwards by caching the last value. A sketch of an
> implementation:
>
> def monotonic(_last=[None]):
>    t = system_clock()  # best effort, but sometimes goes backwards
>    if _last[0] is not None:
>        t = max(t, _last[0])
>    _last[0] = t
>    return t
>
> Overhead if done in Python may be excessive, in which case do it in C.
>
> Unless I've missed something, that guarantees monotonicity -- it may not
> advance from one call to the next, but it will never go backwards.
>
> There's probably even a cleverer implementation that will not repeat the
> same value more than twice in a row. I leave that as an exercise :)
>
> As far as I can tell, "steady" is a misnomer. We can't guarantee that
> the timer will tick at a steady rate. That will depend on the quality of
> the hardware clock.
>
>
> --
> Steven
>
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