Timestamps in kernel tracing with LTTng

Timestamps in kernel tracing with LTTng

[Vince Do]

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Hello,

I was wondering if you can give me more information on how LTTng the
timestamps are taken for a kernel trace, I have read in this article
(Combined Tracing of the Kernel and Applications with LTTng at Linux
Symposium 2009) that TSC and the command rdtsc are used, but the article is
old (2009) and I would like to know if there are been any changes.

Regards,



Vincenzo Do

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Re: Timestamps in kernel tracing with LTTng

[Mathieu Desnoyers]

* Vince Do (blackhole37@gmail.com) wrote:
> Hello,
> 
> I was wondering if you can give me more information on how LTTng the
> timestamps are taken for a kernel trace, I have read in this article
> (Combined Tracing of the Kernel and Applications with LTTng at Linux
> Symposium 2009) that TSC and the command rdtsc are used, but the article is
> old (2009) and I would like to know if there are been any changes.

lttng uses the Linux kernel CLOCK_MONOTONIC clock, read through
clock_gettime() in user-space, and from ktime_get() in the kernel.

On x86 systems where TSC are synchronized and constant, it uses rdtsc,
scales that value to nanoseconds, and adds an offset. The fall-back is
usually the HPET timer on x86.

Thanks,

Mathieu

> 
> Regards,
> 
> 
> 
> Vincenzo Do

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> lttng-dev mailing list
> lttng-dev@lists.lttng.org
> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com