[lttng-dev] tracing multithread user program and API support for enabling/disabling events and for adding/removing context fields

[Yonghong Yan]

We are experimenting LTTng for tracing multi-threaded program, it works
very well for us. Thank you for having this great tool. But we have some
concerns about the overhead and scalability of the tracing. Could you share
some insight of the following questions?
1. The session domain communicates with the user application via Unix
domain socket, from LTTng document. is the communication frequent, such as
each event requires communication, or the communication just happens at the
beginning to configure user space tracing?
2. For the consumer domain, is the consumer domain has a thread per
CPU/channel to write to disk or relay the traces, or it is a single
threaded-process handling all the channels and ring buffers, which could
become a bottleneck if we have large number of user threads all feeding
traces?
3. In the one channel/ring buffer per CPU setting, if a user thread
migrates from one CPU to another, are the traces generated by that user
thread fed to the two channels/buffers for the two CPUs?
4. So far, the events for tracing can be enabled and disabled from command
line, are you considering to have runtime options (APIs) to enable or
disable certain events? Or this is the feature that already in or can be
implemented in different way?
5. For context field, from the document, context fields cannot be removed
from a channel once you add it. I would like to request a feature to allow
removing context fields in the user program.

Thank you very much.
Yonghong
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