On Tue, May 3, 2011 at 5:44 PM, David T. Lewis
<lewis@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