[lttng-dev] lttng enable-channel option for blocking

[Mathieu Desnoyers]

* Woegerer, Paul (Paul_Woegerer at mentor.com) wrote:
> There are use-cases where loosing an event is not acceptable.
>
> Is there some way to make a userspace application block if the buffer is  
> full (instead of discarding the event or overwriting an old event) ?
>
> I'm thinking about something like:
>
> lttng enable-channel myblockingchannel --block
>
> I know I can increase the subbuf-size but sometimes this is not an  
> option (embedded targets with less RAM).
>
> A new channel option like --block would be a fine complement for the  
> already existing options --discard and --overwrite.
>
> What do you think ?

I already thought about permitting this, but we currently don't. The
first thing I must say about this is that I prefer to wait a bit before
we add this feature, and think about its impact thoroughly, because
allowing the tracer to block applications gives a lot of power to the
tracer: e.g., if tracing is stopped due to error conditions, or disk
full, or network traffic slowdown, how do we handle the fact that this
might block progress in all traced applications ?

The current modes (discard and overwrite) let the applications continue
even if there is too much data being recorded into the trace buffers --
this is a "safe" approach.

How would you recommend dealing with the possible pitfalls of blocking
traced applications ? We would need a mechanism in place to ensure
gathering a trace cannot make applications unresponsive.

Thanks,

Mathieu


> Thanks,
> Paul
>
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com