[lttng-dev] babeltrace and strictly sorting events

[Thibault, Daniel]

   Babeltrace sorts the events submitted to it from a variety of source files (multiple traces, each potentially with multiple domains, each potentially with multiple sources (for UST anyway), each potentially with multiple channels, each potentially with multiple CPU IDs) based on timestamps (babeltrace/lib/iterator.c's stream_compare).  This works fine, but what about the case where two events have strictly the same timestamp?  Within a single host, this can only happen if they have different CPU IDs.  It can also happen when merging traces from different hosts.  As it currently stands, stream_compare looks at the timestamps only, so the ordering in the aforementioned cases is undefined (left to the luck of the heap sort).

   I think stream_compare needs to also check for (s_a->parent.real_timestamp == s_b->parent.real_timestamp) and, in that case only, compare hosts (alphabetical ordering is as good as any), then CPU IDs (numerical ordering).  Obviously, this is a potentially expensive change since this function is called awfully often when babeltracing or lttngtopping large traces, so maybe the extra tests could be controlled by a command option (called --strict-ordering, say).

   Or is this so rare it's not worth bothering with?  In which case a small cautionary note in babeltrace's man pages would be enough (those man pages really ought to state that babeltrace's output is time-ordered; right now they don't mention this at all).

Daniel U. Thibault
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