count all events in stream with babeltrace API

Question

I have a LTTNg trace, which i am parsing using babeltrace API. So i was wondering if I could count all events in trace (or stream) without iterating over them. What functions from publilc API I can use to do that ?

Answer 1

The very nature of CTF makes it impossible to count the event records of a given packet in constant time. The packet's context could include an event record count field somehow, but it's not specified, so generic tools would not use it.

Thus the only way to count events is to iterate the event records, unfortunately. The easiest way is to count the number of lines that the text format of the babeltrace(1) tool prints:

babeltrace /path/to/ctf/trace/directory | wc --lines

This works as long as there's one line per printed event record, which is the case unless an event record contains a string field which has a newline (currently not escaped in the text output).

You may also wish to consider discarded event records. They are not printed to the standard output by babeltrace(1), but the tool prints a message including the count to the standard error when they are detected.

There's no way with the current babeltrace(1) tool to only print the event records which belong to the packets of a given data stream. If you need this, what I suggest is that you remove all the data stream files except the one for which you need an event record count, and run the command above again.

Also consider the Babeltrace Python bindings, for example (not tested):

import babeltrace


def count_ctf_event_records(path):
    trace_collection = babeltrace.TraceCollection()
    trace_collection.add_trace(path, 'ctf')
    return sum(1 for event in trace_collection.events)


if __name__ == '__main__':
    import sys

    print(count_ctf_event_records(sys.argv[1]))

Saved as count.py, you can try this:

python3 count.py /path/to/ctf/trace/directory

Counting the event records of a specific data stream with the Python bindings is left as an exercise for the reader.

Having said this, I don't know if the Python bindings approach is faster than the babeltrace(1) one.