[lttng-dev] Overwrite mode, lost events, and reader behaviour

[Thibault, Daniel]

   I'd like a clarification.  In overwrite mode, and assuming the writers are overwhelming the consumers, what is the behaviour?

   Assume we have this situation within a given channel (numbering the sub-buffers in chronological order), and the writers are about to catch up with the reader (the extra sub-buffer is double-bracketed and the sub-buffer it is "shadowing" is marked with a trailing asterisk):

[8] [9] [2] [3] [4] [5] [6] [7] [[-]]   reader at [2], writers at [9], spare sub-buffer unused/unallocated

[8] [9] [10]* [3] [4] [5] [6] [7] [[2]]   reader at [[2]], writers at [10], spare sub-buffer shadowing [10]

[8] [9] [10]* [11] [4] [5] [6] [7] [[2]]   reader at [[2]], writers at [11], spare sub-buffer shadowing [10]

   Now, when the reader completes evacuating [[2]], it moves on to [11] and realises there is a time gap.  I suspect it does not just jump its clock ahead, because that would give up on [4] through [10].  Does it skip to the next sub-buffer until it sees the clock go backward (the circularity of the buffer guarantees there is always one such discontinuity), resuming its work with [4]?  If yes, in the worst case the reader will go once around the sub-buffers, checking timestamps, before resuming its work.  This could be avoided if the buffer metadata kept the spare sub-buffer chained to the oldest one (i.e. the third step is [8] [9] [10] [11]* [4] [5] [6] [7] [[2]]), so the reader never needs to call "next()" more than once.  Is this what lttng already does?

Daniel U. Thibault
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