[time-nuts] Win XP and NIST time

Dan Kemppainen dan at irtelemetrics.com
Wed Mar 27 15:30:52 EDT 2013

On 3/27/2013 2:54 PM, time-nuts-request at febo.com wrote:
>> Then all bets are off if you have a CPU that runs at variable speed if you
>> >want the result to be actual time.
> I think that got fixed on newer CPU chips.  I don't know when.
> Another interesting problem in that area is that the temperature changes with
> the CPU load.  The crystal on most PCs actually makes a pretty good
> thermometer.
My modern processors (Phenom II x4's etc) don't have this issue with the 
performance counters. I believe that has been sorted out for a while 
now. (Thus my qualification of 'should' give you a time stamp from 
system start). I wouldn't use the performance counters in any critical 
code, just for testing said code loops while writing them.

As for affinity, I do lock my time critical loops in windows to a single 
core before starting them. Windows is terrible at time critical data 
streaming stuff when handing off threads from one core to another. You 
can see the problems pop up as soon as the CPU load shifts from one core 
to another. And, don't get me started about hyperthreading!  :)

The bottom line is that windows is not good at doing anything in a 
timely manner. (Thus the use of lots of buffering!)  It was never was 
meant to run time critical applications, and probably never will.

So, knowing what we do about windows, why worry about locking it to 
anything? Even if the core and RTC were locked to a GPSDO, what will you 
gain? Correct time of day within a few seconds is good enough for me!


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