[ntp:questions] WinNT Port Performance Counter Stability and Drift

David Taylor david-taylor at blueyonder.co.uk.invalid
Fri Nov 8 06:58:30 UTC 2013

On 07/11/2013 22:15, Brian Inglis wrote:
>  From the attached extract from my ntp log the current performance
> counter appears to have much higher drift ~25PPM than my hardware clock;
> my own calibration, timings, and loopstats over the last couple of years
> show ~0.9PPM.
> A possible reason is that currently when calibrating and using PCC only
> a bare rdtsc instruction is used.
>  From discussions in various places, summarized well in the article
> linked below, rdtsc may be executed out of order, adding jitter.
> These discussions recommend rdtsc be preceded by mfence (as it works on
> all PCs that support rdtsc) to avoid out of order execution during
> calibration loops.
> The calibration also needs to be wired to a single cpu, results from the
> first call to the calibration function discarded to eliminate cache and
> pipeline fill delays, and all results significantly higher than average
> discarded because the hardware can switch into System Management Mode
> BIOS at random, mainly to handle USB devices like mice, keyboards, drives.
> See
> http://lists.freebsd.org/pipermail/freebsd-amd64/2012-July/014756.html,
> links from that, and similar articles on the LKML and MSDN.

Sounds like one for the NTP Bugzilla


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