[ntp:questions] questions] WinNT Port Performance Counter Stability and Drift
David J Taylor
david-taylor at blueyonder.co.uk
Sat Nov 9 15:26:45 UTC 2013
From: Charles Elliott
Cc: questions at lists.ntp.org
The result of reading the timestamp counter can vary wildly due to EIST
(speed step technology), turbo modes, and owner overclocking, in addition to
differences in CPUs, as noted. There is quite a bit about this on the
Internet. As I recall, most writers recommend not using it, but if one
must, using it only for short interval timing and after repeatedly measuring
the frequency of the counter. The latter can take quite a bit of time, as
it should be done several times, and for different interval lengths, and
taking the average or median of the results.
Most authors recommend using QueryPerformanceCounter and
QueryPerformanceFreq if it can be determined that these functions are tied
to the High Performance Event Timer (HPET), which they are on most modern
systems. I believe that code to do this is already in NTPD. You can tell
this from the messages in the Event Log (on Windows) when NTPD starts up.
NTPD chooses the timer whose frequency is that of the HPET.
Charles, and mail list admins - just to let you know that your message did
not appear to reach the newsgroup:
at least on my feed from Eternal-September.
SatSignal Software - Quality software written to your requirements
Email: david-taylor at blueyonder.co.uk
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