[lttng-dev] Merge two LTTng CTF traces into one

Thu Jan 8 11:42:32 EST 2015

Hi Divya,

I'm putting you back on the lttng-dev mailing list, please reply to that 
list in the future so other people can answer, because I think that 
trace synchronization is not exactly what you are looking for here...

> I tried the sessions at same time and communication events in between 
> and it worked :)
> I am getting quality as accurate and I can see both traces merged .
Excellent!
>
> Now I come at my second  point i.e taking traces from lttng sessions 
> at different machines and at different time and may be communication 
> events or might be not.
If the sessions are taken are different moments, they can _never_ 
synchronize. Trace synchronization first matches events such that one 
single event from a trace (for example "sending a packet") can be 
correlated to exactly one event in the second trace (the reception of 
that packet). If that is the case, the first event can have a timestamp 
of now and the second be timed to January 1st 1970, trace 
synchronization will work and the 2 traces will have a common time 
reference after synchronization.

>
> So In my opinion I need to apply time offset on one of the trace to 
> match the beginning so that they can be synchronized .
>
> So like first trace timestamp starts at 07:13:59 .499 222 870 and 
> other starts at 11:21:35.537 682 885 . Then I can apply timeoffset ( 
> How much offset ? In apply time offset advanced option If reference 
> time is 7:13.... then target time will be 11:21:35... it will 
> calculate the offset ) on one of the trace to match traces .
>
> Is it possible to synchronize the traces then ? Actually I tried these 
> steps but not getting the synchronized traces but then both traces 
> beginning is exactly same ( obviously it will as applied by the offset)
>
> Please let me know how can I synchronize the traces from different 
> machines and at different time
I'm not sure what your end goal is exactly. Do you want to compare trace 
executions? I am cc'ing François Doray who is currently working on a 
trace comparison utility. Here's a presentation he did recently: 
http://ahls.dorsal.polymtl.ca/system/files/Comparison%20of%20Traces%20to%20Diagnose%20Performance%20Variations.pdf

His work is still experimental and available on github. If that's what 
you're interested in, he might give you some information on how to use it.

Otherwise, please explain what you are trying to achieve.

Geneviève

>
>
> On Thu, Jan 8, 2015 at 12:39 AM, Geneviève Bastien 
> <gbastien at versatic.net <mailto:gbastien at versatic.net>> wrote:
>
>     On 15-01-07 11:32 AM, Divya Vyas wrote:
>>
>>     Hi ,
>>
>>     I have some comments below and thanks a lot for your help
>>
>>     On Jan 7, 2015 7:55 PM, "Geneviève Bastien"
>>     <gbastien at versatic.net <mailto:gbastien at versatic.net>> wrote:
>>     >
>>     > Hi Divya,
>>     >
>>     >
>>     > On 15-01-07 01:24 AM, Divya Vyas wrote:
>>     >>
>>     >>
>>     >> Hi Genevieve,
>>     >>
>>     >> My target to is merge the traces of two different machines
>>     taken on different time or same time .
>>     >
>>     > Trace synchronization works by first matching events
>>     corresponding to the sending and the reception of a TCP packet
>>     from both traces, and then taking those matched events and
>>     computing a formula that will transform timestamps from one trace
>>     to the same clock as another trace. So only traces taken at the
>>     same time (meaning together, not the same clock time of course)
>>     will work.
>>
>>     I am not getting this last point , where the traces are taken
>>     together. You mean that we start lttng session on two machines at
>>     around same time and then during  session use some communication
>>     events . Then take the traces into tracecompass for
>>     synchronization in experiment.  I am not getting in which cases
>>     time offset is needed?
>>
>
>     Yes, two sessions on two machines at the same time and they
>     communicate during those sessions. Then you can synchronize
>     traces. The following traces are used in unit tests to test trace
>     synchronization if you wish to see what it looks like:
>     http://archive.eclipse.org/tracecompass/test-traces/ctf/synctraces.tar.gz
>
>>     In what traces I need to apply time offset so that their
>>     beginning matches. Is it applicable on the above case which I am
>>     thinking above.
>>
>
>     You never _need_ to apply time offset. It is if you have traces
>     that do not contain communication events, and you want one of them
>     to be offsetted by a certain time. For example, you have a trace
>     taken GST-5 and one at GST, you cannot synchronize them because
>     they don't have communication events but you want to analyze them
>     with the same time reference, you would offset the one at GST-5 by
>     +5 hours.
>
>     Or if you have a custom event that you know happened at the same
>     time on both traces, but synchronization does not support that
>     event, then you can offset one trace so that both events from both
>     traces have the same timestamp.
>
>     Time offsetting is a manual process by the user, while trace
>     synchronization is an automatic analysis using pre-defined events
>     to compute a formula to transform timestamps from one trace to the
>     same clock as the other.
>
>>     >
>>     >
>>     >>
>>     >> Actually synchronization was not working on different machine
>>     so I tried on same machine.  But now I got it correctly I think so :)
>>     >
>>     > Indeed, you have the right events now, with the net_dev_queue
>>     and netif_receive_skb. So you should be OK. But I *think* this
>>     method works only when on a LAN. If your computers communicate
>>     through internet, you're better off with the
>>     inet_sock_local_[in|out] method
>>
>>     They are connected through LAN only.
>>
>>     >
>>     >>
>>     >> For different time we need to apply offset so that their
>>     beginning matches. I think I need to do this .
>>     >>
>>     >> I tried applying offset on eclipse luna ( I added linux tools
>>     in install new software)   but I am not getting this option "
>>     Apply time offset ".
>>     >
>>     > It is in master only so far, so you should use tracecompass to
>>     have this option.
>>     >
>>     >>
>>     >> I tried on tracecompass also, where I am getting time offset
>>     option but the synchronization view is empty . The window is not
>>     showing anything. The tracecompass version is 0.1.0
>>     >
>>     > The time offset does not involve synchronization at all, it is
>>     just an offset, so you would not see anything in the
>>     synchronization view. But in the "Properties" view, when you
>>     select the trace, the time offset is displayed.
>>
>>     I meant to say that I tried synchronizing the traces in
>>     tracecompass without time offset . The synchronizing view is empty.
>>     It should show at least quality absent etc.
>>
>
>     Quality absent would show only if the traces are from different
>     machines. If you are trying to synchronize traces from the same
>     machine, that view will stay empty.
>
>     Geneviève
>
>
>

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