Re: Timestamp Skew

["Gianluca Varenni" <gianluca.varenni () cacetech com>]

--------------------------------------------------
From: "Guy Harris" <guy () alum mit edu>
Sent: Friday, January 15, 2010 3:20 PM
To: "Community support list for Wireshark" <wireshark-users () wireshark org>
Subject: Re: [Wireshark-users] Timestamp Skew

> On Jan 15, 2010, at 3:04 PM, Gianluca Varenni wrote:
>
> > It's not documented, and depends on the version of Windows, the service
> > pack, and the HAL type... Most of the times it's a non-compensated RDTSC,
> > however.
>
> If it's just RDTSC, wouldn't it have the same problems on "SpeedStep 
> machines" as raw RDTSC?  The description in the mail was

The documentation for KeQueryPerformanceCounter doesn't say anything about 
that. So it's entirely possible that they do something under the hood to 
prevent this issue.

Have a nice day
GV

> > Possible values are
> >   a.. 0 (default) -> Timestamps generated through 
> > KeQueryPerformanceCounter, less reliable on SMP/HyperThreading machines, 
> > precision = some microseconds
> >   b.. 2 -> Timestamps generated through KeQuerySystemTime, more reliable 
> > on SMP/HyperThreading machines, precision = scheduling quantum (10/15 ms)
> >   c.. 3 -> Timestamps generated through the i386 instruction RDTSC, less 
> > reliable on SMP/HyperThreading/SpeedStep machines, precision = some 
> > microseconds
>
> Also, should it say "machines where the clock rate is variable" or 
> something such as that?  I don't know whether Intel still calls it 
> "SpeedStep", and AMD have their own names for it.
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