[lttng-dev] Performance impact using the "filter" option

[Michel Dagenais]

> If I understand correctly, filtering is done at the client
> application side.

With UST yes. 

> This means that filtering could theoretically lower the performance
> of the running application which has to check, for each event,
> whether it should be sent to the daemon or not.

Each event is checked against the filter and then written to the buffer or not. With a simple filter and most events filtered out, checking the filter is faster than writing to the buffer (with atomic operations to increment the buffer counters) and you definitely save (at each tracepoint and in the daemon with trace buffers filling more slowly). If the filter is complicated and most often keeps the event, then you certainly loose. 
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