[lttng-dev] Timestamps in kernel tracing with LTTng

[Mathieu Desnoyers]

* Vince Do (blackhole37 at 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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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com