* Matthew Khouzam ([email protected]) wrote: > Hi tracing werewolves, > > I want to clear up some stuff about lost events. > > Lost events are caused, in lttng 2.x when the tracer does not have > enough memory available to write an event. This can be due to: > * events coming in too fast (faster than we can write them) > * a given event being too large > * we have an event that does a nested wrap around > > LTTng will not write the whole packet that is lost.
LTTng does not write an _event_ discarded at all. However, a packet "lost" is a different thing: it only happens in overwrite mode, when we overwrite a packet. > It uses a ring > buffer and the packet will just be overwritten in the buffer, but a > counter will be incremented. For an _event_ discarded, we're not even writing it into the buffer. > The counter is written in the packet header in the CTF trace. Indeed, the event lost counters are written in the packet in the ctf trace. > > When reading a trace, if you just read the packet headers, you will see > something as follows: > > Packet 1: > ts_begin: 100 > ts_end: 200 > discarded: 0 > > Packet 2: > ts_begin: 300 > ts_end: 400 > discarded: 100 > > Packet 3: > ts_begin: 500 > ts_end: 600 > discarded: 100 > > Packet 4: > ts_begin: 650 > ts_end: 750 > discarded: 200 > > With this example, the lost events are located between packets 1, 2 and > packets 3,4. > > If the events are too large, they are discarded in the current packet, > lttng does not split events into packets (yet?), and this is lost during > a current packet. > > That means if the events are lost between 1 and 2, they are potentially > in the range of [200-400] but probably in [200-300] > The next lost event are between 3 and 4 (events discarded is a total > count, not local to that packet), so 100 events are also lost between > [600-750] but likely in between [600-650]. If you can confirm that all > events WITH CONTEXTS are smaller than the packet sizes, I think it's a > shoe in that it's between the packets and not containing the next packet. "and not contained in the next packet". Thanks, Mathieu > > Hope this clarifies some issues, it did for me! > > Matthew > > _______________________________________________ > lttng-dev mailing list > [email protected] > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list [email protected] http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
