Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.java.programmer > #37473

Re: Is time an illusion?

Show all headers | View raw

On 2017-09-26 00:02, Eric Sosman wrote:
> On 9/25/2017 4:30 PM, Daniele Futtorovic wrote:
>> Cheers,
>>
>> I'm trying to synchronise a process between two machines (actual, not
>> virtual). I require sub-millisecond precision.
>>
>> I initialise clocks backed by System#nanoTime() on each end and
>> synchronise them somehow (there'll be some deviation at this point.
>> Doesn't matter). Subsequently, I time my process on each end relative to
>> this starting point.
>>
>> I have the impression that I'm seeing time drift in this process.
>> Disregarding measurement errors, the differences between the clocks seem
>> to increase over time.
>>
>> Q: Is this to be expected? I've been trying to read up on it a bit
>> (Oracle memory-holing all the sun.com blogs doesn't help), but wanted
>> some confirmation: is time really an illusion? Are 10^9 nanoseconds on
>> box A possibly not 10^9 nanoseconds on box B?
> 
>     It's expected.  If the oscillators underlying the two machines'
> clocks run at rates that differ by only 0.001% -- one part in a
> hundred thousand -- the clock readings will diverge by 860-odd
> milliseconds per day.
> 
>> If so, seriously, what the heck? I'm aware I might have stumbled on a
>> very elementary truth here after so many years in the business, but I
>> must say, the idea itself feels deeply unsettling.
>>
>> Existential musings aside, would any of you have suggestions as to how
>> tackle the issue?
> 
>     http://www.ntp.org/
> 
>     "Serious" systems already have NTP implementations, so it may just
> be a matter of enabling the service on both machines and making sure
> firewalls and such don't prevent them from reaching public time servers.
> 
>     I'm no expert on this, but I think that if the two machines run
> NTP independently they may adjust themselves at different moments,
> leading to disagreements of a different sort.  The solution (I'd
> imagine, as a non-expert) would be to have one machine sync externally
> and then turn around and act as the time server for the other.

I don't think NTP would be a good fit for me. If I have to adjust clocks
periodically anyway, I can do that within my setup; I just need to elect
one end as the master and go from there. But is there perhaps something
I could tap, even if I have to go native, or do I really have to accept
that I just can't ever have congruent time intervals across potentially
heterogeneous hardware?

Also, if there is a drift, can I expect that drift to be constant? That
is, if I observe that box A ticks, say, 1% faster than box B over a
given interval of time, can I assume that it will do so for /any/ given
interval of time? I guess not, right?

-- 
DF.

Back to comp.lang.java.programmer | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-25 22:30 +0200
  Re: Is time an illusion? Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-09-25 18:02 -0400
    Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-26 00:16 +0200
      Re: Is time an illusion? Jerry Stuckle <jstucklex@attglobal.net> - 2017-09-25 19:18 -0400
      Re: Is time an illusion? Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-09-26 15:37 -0400
        Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-27 01:13 +0200
          Re: Is time an illusion? Jerry Stuckle <jstucklex@attglobal.net> - 2017-09-26 21:00 -0400
            Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-27 04:26 +0200
              Re: Is time an illusion? Jerry Stuckle <jstucklex@attglobal.net> - 2017-09-26 23:32 -0400
                Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-28 10:58 +0200
                Re: Is time an illusion? Jerry Stuckle <jstucklex@attglobal.net> - 2017-09-28 12:50 -0400
            Re: Is time an illusion? Eric Douglas <e.d.programmer@gmail.com> - 2017-09-27 05:25 -0700
            Re: Is time an illusion? Arne Vajhøj <arne@vajhoej.dk> - 2017-09-30 22:57 -0400
  Re: Is time an illusion? Arne Vajhøj <arne@vajhoej.dk> - 2017-09-25 20:22 -0400
  Re: Is time an illusion? Marcel Mueller <news.5.maazl@spamgourmet.org> - 2017-09-26 20:18 +0200
    Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-27 01:30 +0200
      Re: Is time an illusion? lewbloch@gmail.com - 2017-09-28 10:52 -0700
  Re: Is time an illusion? Eric Douglas <e.d.programmer@gmail.com> - 2017-09-26 12:59 -0700
    Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-27 01:42 +0200
  Re: Is time an illusion? lewbloch@gmail.com - 2017-09-28 10:49 -0700
    Re: Is time an illusion? Robert Klemme <shortcutter@googlemail.com> - 2017-09-28 22:33 +0200

csiph-web