[Python-Dev] PEP 564: Add new time functions with nanosecond resolution (original) (raw)

Victor Stinner victor.stinner at gmail.com
Tue Oct 24 03:00:45 EDT 2017

Thanks Thomas, it was interesting! You confirmed that time.time_ns() and other system clocks exposed by Python are inappropriate for sub-nanosecond physical experiment.

By the way, you mentionned that clocks are not synchronized. That's another revelant point. Even if system clocks are synchronized on a single computer, I read that you cannot reach nanosecond resolution for a NTP synchronization even in a small LAN.

For large systems or distributed systems, a "global (synchronized) clock" is not an option. You cannot synchronize clocks correctly, so your algorithms must not rely on time, or at least not too precise resolution.

I am saying that to again repeat that we are far from sub-second nanosecond resolution for system clock.

Victor

Le 24 oct. 2017 01:39, "Thomas Jollans" <tjol at tjol.eu> a écrit :

On 22/10/17 17:06, Wes Turner wrote: > There are current applications with greater-than nanosecond precision: > > - relativity experiments > - particle experiments > > Must they always use their own implementations of time., datetime. > init, fromordinal, fromtimestamp ?! > > - https://scholar.google.com/scholar?q=femtosecond > - https://scholar.google.com/scholar?q=attosecond > - GPS now supports nanosecond resolution > -

Sure, but in these kinds of experiments you don't have a "timestamp" in the usual sense. You'll have some kind of high-precision "clock", but in most cases there's no way and no reason to synchronise this to wall time. You end up distinguishing between "macro-time" (wall time) and "micro-time" (time in the experiment relative to something) In a particle accelerator, you care about measuring relative times of almost-simultaneous detection events with extremely high precision. You'll also presumably have a timestamp for the event, but you won't be able or willing to measure that with anything like the same accuracy. While you might be able to say that you detected, say, a muon at 01:23:45.6789 at Δt=543.6ps*, you have femtosecond resolution, you have a timestamp, but you don't have a femtosecond timestamp. In ultrafast spectroscopy, we get a time resolution equal to the duration of your laser pulses (fs-ps), but all the micro-times measured will be relative to some reference laser pulse, which repeats at >MHz frequencies. We also integrate over millions of events - wall-time timestamps don't enter into it. In summary, yes, when writing software for experiments working with high time resolution you have to write your own implementations of whatever data formats best describe time as you're measuring it, which generally won't line up with time as a PC (or a railway company) looks at it. Cheers Thomas

* The example is implausible not least because I understand muon chambers tend to be a fair bit bigger than 15cm, but you get my point.

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