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

[Thibault, Daniel]

-----Message d'origine-----
Envoyé : 20 novembre 2013 09:44

> We don't use the timestamps for this. Each packet has a sequence number given by the producer, and the reader is querying for packets by requesting sequence numbers
> (increasing by one each time). In the case you present here, the reader will try to query the sequence number of an overwritten packet, and will therefore not be able to get it.
> It will move on to the next packet until it finally finds the first packet which sequence number matches its current sequence number (or the writer position).

   So my original suggestion holds: performance could increase just a little bit (on the reader side) if the reader's request (lib_ring_buffer_get_subbuf, is it?) did not return -EAGAIN but rather the current "earliest available sequence number" it should next ask for.  In the (presumably rare) case where the reader has been passed by the writers by several sub-buffer sequence numbers' worth, this would allow the reader to skip ahead to the next likely-to-succeed sequence number request.  Does this make sense?

Daniel U. Thibault
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