[lttng-dev] Discarded events

[Erica Bugden]

Hello Bala,

If you'd like more information about which event types (tracepoints) are 
practical for investigating which areas, the `ltting-utils` [1] project 
could be useful! The repository contains scripts that record LTTng 
traces. Each script is focused on a different investigation area (e.g. 
network, interrupts, disk, etc).

You could use the scripts themselves and/or read the `*.profile` files 
[2] to see the event types enabled by the different profiles. For 
example, in the disk analysis profile (disk.profile), the following 
event types are enabled:

     - block_rq_complete
     - block_rq_insert
     - block_rq_issue
     - block_bio_frontmerge

As Millian suggested, it is often best to only trace the event types you 
need for the type of issue you are investigating since this increases 
the accuracy of the data collected (less tracing overhead, fewer dropped 
events) and also makes the trace easier to understand (less data volume, 
less data noise).

Best,

Erica

----

LINKS

[1] lttng-utils: https://github.com/tahini/lttng-utils/tree/master
[2] Tracing Profiles: 
https://github.com/tahini/lttng-utils/tree/master/lttngutils/profiles

On 2023-08-15 01:24, Milian Wolff via lttng-dev wrote:
> On Sonntag, 13. August 2023 16:58:17 CEST Bala Gundeboina via lttng-dev wrote:
>> Hi,
>>
>> I am Using LTTng in my project , I am recording a kernel session for long
>> duration (eg more than 15minutes) I am enabling all the events for
>> recording but some events discarding after stopping the session, if i run
>> more than 5 minutes only some events are discarding. I have tried to change
>> in the .ltttngrc file also to increase buffer-size.
> 
> While not directly related to your question, I strongly encourage you to
> review the list of events and only enable those that are actually required for
> the specific issue you are investigating. Some kernel events are _extremely_
> high frequency but have often no practical use for many (at least in my
> experience and for my own purposes). Examples for the latter are esp. the
> kmalloc tracepoints.
> 
>> lttng destroy
>> Destroying session `my-kernel-session`...
>> Session `my-kernel-session` destroyed
>> Warning: 94424667 events were discarded, please refer to the documentation
>> on channel configuration
>>
>> actually when i am running below command i am getting below warning i am
>> thinking this is the issue because sometimes this warning is not coming
>> that i didn't seen discarded events.  how to overcome this problem can you
>> provide some detailed lttng-sessiond --daemonize
>> Warning: Failed to produce a random seed using getrandom(), falling back to
>> pseudo-random device seed generation which will block until its pool is
>> initialized: Failed to get true random data using getre
>>
>> Thanks & Regards
>> Bala Gundeboina
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>