LTTNG: using with a popular library

Question

I have a trivially simple question that I could not find the answer yet.

Say, I have a shared library X that is used by 100 simultaneously running applications A0, A1, ... A99. I had instrumented my library and with LTTNG using "X-Provider" as the provider name. How can my user distinguish between X-Provider events that happen in A10 and A15?

Answer 1

With the lttng command-line utility, add the vpid context field to be recorded to your event records, for example:

$ lttng add-context -u -t vpid

This targets all the user space channels of the current recording session; you can of course select a specific recording session and/or channel (see lttng-add-context(1)).

You need to use this command before you start the tracers (lttng start).

All your event records will have a context field named vpid which is the virtual IDs of the processes which emitted them.

Now, you need to find a way to associate such VPIDs to instance IDs (A10, A15, etc.). You can do so with your own application-specific strategy, or you can simply record an initial event when starting the application which indicates the instance ID/name, for example:

tracepoint(x_provider, app_state, app_id);

Then, when reading the resulting trace(s), you can correlate the x_provider:app_state event record info with the records of your library-emitted events. For example, using the Babeltrace 2 Python bindings:

import bt2

vpid_app_ids = {}

for msg in bt2.TraceCollectionMessageIterator('/path/to/trace'):
    if type(msg) is not bt2._EventMessageConst:
        continue

    vpid = int(msg.event.common_context_field['vpid'])

    if msg.event.name == 'x_provider:app_state':
        app_id = int(msg.event.payload_field['app_id'])
        vpid_app_ids[vpid] = app_id
    else:
        app_id = vpid_app_ids[vpid]

        # Continue to process `msg.event` knowing the instance ID is
        # `app_id`...

I didn't test the example above, but you get the gist.