Re: [Xenomai-help] TSCs synchron on multi-core CPUs?

[Jan Kiszka <jan.kiszka@domain.hid>]

Karl Reichert wrote:
>> Hi everybody,
>>
>> I have a question concerning the time stamp counters of the Intel
>> core2duo/quad CPUs.
>> They will be used e.g. with the Xenomai native skin call rt_time_tsc which
>> ends up in
>> an assembler statement "rdtsc".
>> My question is now:
>> Are the values of TSC synchron with dual/quad core CPUs? Or is there a
>> possible drift between
>> the CPUs?
>> Background of the question is:
>> I want to measure the time of a event that occurs within one task.
>> A second event is checked in another task on a different core. The time
>> difference between the
>> events has to be checked.
>> Can this be done precisely with rdtsc() or is a mechanism required that
>> uses the TSC of exactly one 
>> core?
>>
>> Thanks for all hints on that question!
>>
>> Regards
>>
>> Mathias
> 
> Hi Mathias,
> 
> TSC of the cores may differ: http://en.wikipedia.org/wiki/Time_Stamp_Counter

And if they do on your box, clocktest from the testsuite can tell you.

Note that Xenomai currently relies on TSC being synchronized, or that
you switch to a TSC-less CPU type in your kernel config. Once this
becomes and actual problem for a use case, one may consider adding HPET
as clocksource for Xenomai. So far this wasn't the case.

Jan

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