Hi Mathieu,

I have few questions related to the questions above and please forgive me for my ignorance in advance.

1. When LTTng creates the buffers lets say per uid, the files that show up in LTTng folder, are those files that LTTng has stopped writing in? What I mean is, are those files still being written to or they are safe to be moved.

2. If I want to collect traces lets say every 5 mins for the last 5 mins while LTTng keeps running. What is the best approach? Do I just run BabelTrace on the folder and remove all files from the folder to create space for next files and keep repeating?

3. I understand that I should delete the corresponding .idx file for each log file that I remove, what about metadata file? Do I remove it or leave it as is?

4. The trace file produced by Babeltrace, does it guarantee that the file has traces from all the users and the core till the time stamp of the last line?'

For example if the last line of the trace file looks like this

23:00:00.912089772,...,...

Does that mean that the trace file has all the traces until that time?

Thanks

Ammar

On Sat, Feb 20, 2016 at 10:17 AM, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

Hi,

LTTng implements per-cpu buffers to eliminate false-sharing between
CPUs. Babeltrace and Trace Compass implement iterators that allow
doing the merge of those per-cpu buffers into a single stream on
the fly. Users of Babeltrace and Trace Compass should not have to
worry about doing this merge.

Since LTTng is inherently multi-buffers, we also use this buffer
merging scheme to deal with tracefile rotation, multi-channel, and
combining traces gathered from various processes/user IDs/kernel vs
userspace.

So my question here is: why you say it is much more difficult for
you to deal with those multiple buffers ? Since there are already
libraries in Babeltrace that do that for you, I would expect this
to be a non-issue.

Although it would be technically possible to implement a ring
buffer shared across CPUs, it would be inefficient, and we would
need a good justification for adding this complexity to the
tracers.

Thanks,

Mathieu

----- On Feb 19, 2016, at 4:00 AM, Jeffrey Chen cpthk@hotmail.com wrote:

> Thanks.
> If we make changes to LTTng to add this feature, would it violate the LTTng
> design principle in any way? Thanks.
>
>
> ________________________________________
> From: Philippe Proulx <eeppeliteloop@gmail.com>
> Sent: Tuesday, February 16, 2016 5:02 PM
> To: Jeffrey Chen
> Cc: lttng-dev@lists.lttng.org
> Subject: Re: [lttng-dev] LTTng buffer mode
>
> No you cannot.
>
> There's no hidden global buffering scheme option for user space tracing.
>
> From the LTTng documentation:
>
>> In the user space tracing domain, two buffering schemes are available
>> when creating a channel:
>>
>> Per-PID buffering: keep one ring buffer per process.
>> Per-UID buffering: keep one ring buffer for all processes of a single user.
>
> and:
>
>> The Linux kernel tracing domain only has one available buffering scheme
>> which is to use a single ring buffer for the whole system.
>
> Phil
>
> On Tue, Feb 16, 2016 at 7:55 PM, Jeffrey Chen <cpthk@hotmail.com> wrote:
>> Hi LTTng community:
>>
>>
>> I have a question about LTTng buffer mode. According to documentation, LTTng
>> supports 2 buffer modes, per-pid, and per-uid. Is there any mode to have a
>> global buffer that does not have separate buffers? The reason I asked is
>> because, LTTng writes different buffer traces to different directory/file.
>> It is much more difficult for us to post-process separate directories. We
>> would like to have all traces write into one single buffer and
>> directory/file, so it is easier to do post-processing. Thanks.
>>
>>
>>
>> _______________________________________________
>> lttng-dev mailing list
>> lttng-dev@lists.lttng.org
>> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
>>
>
> _______________________________________________
> lttng-dev mailing list
> lttng-dev@lists.lttng.org
> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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