[Pharo-project] System time

Andres Valloud avalloud at smalltalk.comcastbiz.net
Mon May 2 12:49:32 CEST 2011


I'd look into how the VM knows what time it is.  Sooner or later there 
will be a primitive or some FFI call that asks the OS to tell you the 
time.  You should follow the VM Code Lawyer's Standard Procedure(TM):

1.  What is the VM doing, exactly?

2.  What are the relevant *official* specifications governing what the 
VM is doing?  Be thorough in your search.

3.  Does the VM implementation comply 100% with the specifications' 
requirements?

If you cannot answer "yes" at step 3, then you don't know what's going 
on.  Chances are the VM merely *appears* to work, but you can't quite 
explain why.  This is bad and should be rectified.

If you are forced to answer "no" at step 3, then you know the VM is 
Broken(TM).  A lot of bugs are caused by specification violations.  I 
know working with C specifications is not cool because it's not 
Smalltalk and all of that stuff, but I think it's even less cool to have 
a Broken(TM) VM so something has to be done.

I did some investigation about similar problems for Windows not long 
ago.  Basically, relying on the function QueryPerformanceCounter() is 
just bad.  Moreover, if the x86 VMs depend on the CPU's timestamp 
counter, then you are going to get problems like those you get with 
QueryPerformanceCounter().  Trace through the documentation on the 
function QueryPerformanceCounter(), the boot switch /usepmtimer, and 
something called HPET.

On 5/2/11 0:49 , Mariano Martinez Peck wrote:
>
>
> On Mon, May 2, 2011 at 12:56 AM, Andres Valloud
> <avalloud at smalltalk.comcastbiz.net
> <mailto:avalloud at smalltalk.comcastbiz.net>> wrote:
>
>     Does the Windows VM depend on QueryPerformanceCounter()?
>
>
> I have no idea what that is, but a quick
>
> grep -n "QueryPerformanceCounte" -r
> ~/Pharo/vm/git/cogVMMaster/blessed/platforms
>
> tell me that not.
>
>
>
>     On 5/1/11 16:53 , Alexandre Bergel wrote:
>
>         Strange. I cannot produce this problem on OSX.
>
>         Alexandre
>
>
>         On 30 Apr 2011, at 22:15, Jimmie Houchin wrote:
>
>             Hello,
>
>             My OS for this application is Windows. On a freshly opened
>             image, the image and OS time are in sync. Over a period of
>             time the image drifted 34 seconds away from OS time. I do
>             not know how long I had this image open, but it has been a
>             number of days, possibly a week plus.
>
>             I saved the image and quit. I reopened the image and did
>             DateAndTime now and it was in sync with the OS once again.
>             Same image, vm and OS that was out of sync by 34 seconds
>             only one minute before.
>
>             I have opened an image on my server which is also running
>             the latest cog vm, the current Pharo 1.3 and Windows Vista.
>             I am going to periodically do a DateAndTime now to log the
>             drift if any. Prior to opening this image I updated the OS
>             system clock with time.nist.gov <http://time.nist.gov> which
>             is about 1000 miles away.
>
>             I know that within a second or so it can be subject to my
>             coordination of doing DateAndTime now while watching my OS
>             system clocks seconds. But 34 seconds is beyond any
>             subjective standards.
>
>             The only reason I discovered this was while exploring how I
>             want to retrieve my data, act upon my data and then pause
>             (Delay wait) until time to repeat.
>                 dtnow := DateAndTime now.
>                 dtnext := DateAndTime year: 2011 month: 4 day: 30 hour:
>             11 minute: i second: 5. "The next minute in the future"
>                 (Delay forDuration: (dtnext - dtnow)) wait.
>
>             I got an error, that Duration cannot be a negative number.
>             As I processed the debugger, I saw that dtnow was greater
>             than dtnext. I watched the OS clock and manually selected
>             the next minute before I executed. However, I was within
>             that 34 second window.
>
>             So now I am exploring the problem.
>
>             Thanks.
>
>             Jimmie
>
>             On 4/30/2011 5:06 PM, Alexandre Bergel wrote:
>
>                 'Time now' gives me the same result than OSX
>
>                 Alexandre
>
>
>                 On 30 Apr 2011, at 16:15, Jimmie Houchin wrote:
>
>                     On 4/30/2011 11:50 AM, Stéphane Ducasse wrote:
>
>                             I am developing a time sensitive
>                             application. I poll a server every minute
>                             for new data. The data is time sensitive and
>                             the sooner I can get the data to the release
>                             of the data the better.
>
>                             I have noticed that Pharo's system time
>                             drifts away from the OS system time. I would
>                             really like the Pharo system time to be the
>                             correct time.
>
>                         do you have a repeatable scenario?
>                         VM
>                         Image information.
>
>                     Pharo 1.3 and the latest Cog.
>
>                             I have not been able to find where in Pharo
>                             I can call to update the System clock to the
>                             correct time.
>
>                         you can use OSProcess (only mac and linux)
>
>                     No, that's not the problem. The OS clock is right.
>                     It's Pharo that has drifted away from correct time.
>                     Regardless it would be nice if I have to correct the
>                     OS clock, it would still be nice to resync Pharo
>                     with the OS, keep both in sync and correct. What I
>                     am wanting is simply to resync the Pharo clock with
>                     the OS clock.
>
>                     It seems that when the Pharo image is started that
>                     it gets the OS System time, but keeps its own time
>                     after that. Now I do not know the consequences of
>                     Pharo adjusting its time backward. If this isn't
>                     possible, then I might have to do some workarounds
>                     to insure that I get my data in a timely manner.
>
>                     The image I have been developing in is currently
>                     about 30 seconds ahead of the OS clock. It is on my
>                     laptop. It might be due to the laptops sleep and
>                     hibernation. I have not tested this on my server.
>
>                     That is the situation as best I know it.
>
>                     Thanks.
>
>                     Jimmie
>
>
>
>
>
>
>
>
> --
> Mariano
> http://marianopeck.wordpress.com
>



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