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

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

[M. Koehrer]

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

-- 
Mathias Koehrer
mathias_koehrer@domain.hid


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

[Karl Reichert]

> 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

Best regards
Karl
-- 
von Karl Reichert

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

[Jan Kiszka]

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

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux