[lttng-dev] babeltrace : Clock "monotonic" offset differs between traces

[Jérémie Galarneau]

On Mon, Aug 19, 2013 at 10:32 AM, Thuya Aung <thuyaaung2002 at yahoo.com> wrote:
> Hello lttng-dev,
>
> First of all, I like to thank LTTng development team for such a wonderful tracing tool for us to use.
>
> I am having the following messages when I viewed the trace file in babeltrace.
> [warning] Clock "monotonic" offset differs between traces (delta 52116 ns). Using average.
> [warning] Clock "monotonic" offset differs between traces (delta 72396 ns). Using average.
>
> My questions are,
> 1. Why am I having these warning? Can I reliably analyze traces after having these warning?
> 2. How can I minimize or eliminate that kind of offset?

The discrepancy may be caused by a number of factors.

The clock offset is sampled when the session is started ("lttng
start"). That means that each application being traced will sample the
correlation between UTC and a monotonic clock. The applications may
report a different offset when the system is under heavy load. Note
that this mostly affects user space tracing.

This behavior was changed as of the 2.2 release. The session daemon
now handles the UTC/monotonic correlation sampling. That means
preemption is much less likely to cause the offsets to differ.

The clocks may also lack precision on your particular kernel/architecture.

I'd encourage you to start by upgrading to 2.2 and see if you still
encounter the problem.

As for the reliability of the traces, it entirely depends on what you
are trying to observe/measure. Just be aware that events from
different apps may not appear in the correct order as the time stamps
are not based on the same clock. The difference between the clocks'
offsets lets you know how "off" the events can be off from each other.

Regards,
Jérémie

>
> The version of LTTng tools we are using are,
> lttng-modules : 2.1.1
> lttng-tools   : 2.1.1
> lttng-ust     : 2.1.2
> userspace-rcu : 0.7.6
> babeltrace    : 1.0.3
>
> kernel version : 2.6.37.6 for power-pc
>
> Best Regards,
> Thuya
>
>
>
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-- 
Jérémie Galarneau
EfficiOS Inc.
http://www.efficios.com