[Development] QtCS 2017 logging/tracing session notes

Arnaud Clère arnaud.clere at minmaxmedical.com
Wed Oct 18 09:29:38 CEST 2017

Hi Christian,
Thanks for the pointers regarding LTT and the underlying Common Trace Format, that was very informative.
The fact that CTF supports variants and dynamically sized structures makes it a possible output for modmedLog, as is IETF's Compressed Binary Object Representation.

What is not possible is to use Bind<T> templates to generate a "static" TSDL schema of T, that is, without "runtime" variants. For instance a Bind<QSharedPointer<T>>::bind() will not call Bind<T>::bind() if null. So, the corresponding TSDL must use a variant to account for this possibility. More importantly, a Bind<QVector<T>> would be described as a sequence of variant of either : record | sequence | uint8_t | etc. Also a Bind<MyClass> would be described in the same way as a "generic" Bind<QStringMap<T>> putting all MyClass member names into the trace instead of describing them externally in the TSDL.

This less compact binary representation brings a lot of flexibility in that it would allow reusing existing qDebug tracepoints in the new tracing facility as long as the traced data has a Bind<T> defined (which would be the case for basic Qt types). On the other hand, the impact on performance should be reasonable. For instance, if added keys take 1/3 of your trace volume, I would expect the trace throughput to be 2/3 of the throughput of a trace fully described by a "static" TSDL.

Do you think the tradeoff would be acceptable?

I still need to figure out the best way to insert the tracepoint metadata from qDebug-like tracepoints into CTF. In our TSV+JSON format, we eliminate metadata redundancy on the fly so it does not take more space than actual event data. I can see that CTF has metadata packets for that, so it should be possible to do the same.

Regarding converting large traces to JSON, I agree it is not appropriate for your use case. The idea is to agree on a common logical data format based on JSON to allow supporting more physical formats (such as CTF) in the future. But I cannot tell when such support would be available.

Regarding access to forge.imag.fr repository. Did it eventually work? Otherwise, I'll mention that to the admin team.

Thanks a lot!

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