[lttng-dev] Request information on "Live" view of traces

Julien Desfossez jdesfossez at efficios.com
Tue Sep 10 11:28:53 EDT 2013

On 13-09-10 09:20 AM, Thibault, Daniel wrote:
> -----Message d'origine-----
> From:   Julien Desfossez <jdesfossez at efficios.com>
> The live trace reading mechanism offers guarantees that the viewer can never be in a position where it has to assume that it can read the trace safely. 
> ------------------------------
>    Could you clarify what you mean by that?  What does "assuming that the trace can be read safely" entail that would be bad?
There are two risks when reading an active trace : reading a trace
without the complete metadata, and assuming a stream is inactive.

If the viewer tries to read a trace for which it does not have the full
metadata (which is not known during normal tracing), it risks to crash
reading the trace. The live trace reading feature prevents the viewer
from getting trace packets if there is new metadata appended between two
packets read.

Also, if a stream is inactive, usually the viewer would have to wait
until it is active again to continue processing the trace. The live
trace reading feature informs the viewer that it can safely consider a
stream inactive up to a certain point in time. That way, it can continue
to process the trace without waiting indefinitely for an inactive
stream. The live-timer parameter, configured during the session creation
by the user, defines the sending rate of these inactivity beacons.

I hope it clarifies the situation.



> Daniel U. Thibault
> Protection des systèmes et contremesures (PSC) | Systems Protection & Countermeasures (SPC)
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