David T. Lewis 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.
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
I'm just starting in Smalltalk, so I'll make some very generic comments.
1. Could the squeak exe process get a system interrupt between when it
reads the time from windows and when it it adds a ticks to its clock.
2. VMWare is a completely different form of virtual machine from
Squeak, however interesting reading none the less. More than you
wanted to know about system time:
http://www.vmware.com/files/pdf/Timekeeping-In-VirtualMachines.pdf
3. Idea from (2.) possibly test the effect of running QuickTime in
parallel with Squeak?
Ben
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