[Vm-dev] Fwd: [Pharo-project] System time

[Eliot Miranda]

On Tue, May 3, 2011 at 5:44 PM, David T. Lewis <lewis at mail.msen.com> wrote:

>
> On Tue, May 03, 2011 at 04:02:04AM +0200, Levente Uzonyi wrote:
> >
> > On Mon, 2 May 2011, David T. Lewis wrote:
> > >
> > >I have not looked at this carefully, but I suspect an issue in
> Chronology
> > >(as opposed to a VM or primitive issue). DateAndTime initializes some
> > >cached
> > >values at image startup time, and thereafter calculates things based on
> the
> > >millisecond clock. If the millisecond clock drifts with respect to
> system
> > >time
> > >on the underlying platform (as presumably would be the case if the
> system
> > >clock
> > >is synchronized using ntp), then DateAndTime's opinion about current
> time
> > >will
> > >drift with respect to the underlying platform.
> >
> > The amount of the drift seems to be 1 millisecond every 14-16 seconds on
> > my pc. I wrote some code to get the system time via FFI[1], then ran the
> > following:
> >
> > | data |
> > data := (1 to: 100) collect: [ :run |
> >       1 seconds asDelay wait.
> >       { DateAndTime now. Win32SystemTime localDateAndTime } ].
> > (data collect: [ :each | each first - each second ] as: Set) sorted
> explore
> >
> >
> > Levente
> >
> > [1] http://leves.web.elte.hu/squeak/Win32SystemTime.st
>
> I am now running a similar test (attached) on Linux. I was expecting
> to see clock drift exactly the same as that reported on Windows, but
> so far after about 15 minutes I see no obvious signs of drift. I
> cannot explain this but I'll leave it running for a while and see
> if anything interesting happens.
>

Its because on linux & mac the clock is derived from gettimeofday which is
wall time but on Windows it is derived from timeGetTime which isn't.  The
problem is the lack of a high-resolution wall time on Windows.
 GetSystemTimeAsFileTime et al have something like 16 ms resolution whereas
timeGetTime has 1ms resolution.


>
> Dave
>
> sample output:
>
> === Started at 2011-05-03T20:24:35.03-04:00 ===
> Test duration 0:00:15:46.01
> OS time now: 2011-05-03T20:40:21.039466-04:00
> Squeak time now: 2011-05-03T20:40:21.04-04:00
> Difference: 0:00:00:00.000534
> Drift: 0:00:00:00.000075
> === Started at 2011-05-03T20:24:35.03-04:00 ===
> Test duration 0:00:15:49.072
> OS time now: 2011-05-03T20:40:24.101675-04:00
> Squeak time now: 2011-05-03T20:40:24.102-04:00
> Difference: 0:00:00:00.000325
> Drift: -0:00:00:00.000134
> === Started at 2011-05-03T20:24:35.03-04:00 ===
> Test duration 0:00:15:52.102
> OS time now: 2011-05-03T20:40:27.131382-04:00
> Squeak time now: 2011-05-03T20:40:27.132-04:00
> Difference: 0:00:00:00.000618
> Drift: 0:00:00:00.000159
> === Started at 2011-05-03T20:24:35.03-04:00 ===
> Test duration 0:00:15:55.136
> OS time now: 2011-05-03T20:40:30.165711-04:00
> Squeak time now: 2011-05-03T20:40:30.166-04:00
> Difference: 0:00:00:00.000289
> Drift: -0:00:00:00.00017
>
>
>
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