On 22 May 2018 at 11:16, Vlad <v...@demoninsight.com> wrote: > Greetings, > > I have what I hope is a related question: > > - my use case is: > - I use LTTng for userspace events only (at this time); > - I have the system clock (CLOCK_REALTIME) driven by a PTP driver that > comes with a NIC (SolarFlare) that slaves the system to an externally > provided high-grade PTP master; > - the NIC stamps all arriving IP packets at the hardware level and the > driver then makes the timestamps available in userspace along with each > packet; > - I would like to be able to correlate these timestamps with LTTng > timestamps on the same system. > > - is there a way to configure LTTng to use a user-specified CLOCK_* type? > I understand that that would de-cohere the timestamps from other > interesting timestamps (kernel events, etc) but my case above is valuable > enough that I’d be willing to forgo everything else. >
Hi Vlad, lttng-ust's clock plug-in interface seems like a good fit for what you want. Have a look at the example in lttng-ust's source tree: https://github.com/lttng/lttng-ust/tree/master/doc/examples/clock-override Thanks! Jérémie > > Thank you in advance, > Vlad > > On Apr 20, 2018, at 4:53 PM, Mohamad Gebai <mge...@suse.de> wrote: > > LTTng uses the monotonic clock [1] to timestamp the event at trace-time. > The monotonic clock is more appropriate than the real time (CLOCK_REALTIME) > when you want to timestamp/compare/order certain events within the same > system relatively to each other. Using the real time clock would cause a > problem if say someone (or something, like ntp, or kvm in a VM) changed the > system's time while tracing. > > With that being said, the monotonic clock doesn't give you a time that is > very meaningful as a human (its reference is the system boot). Having the > timestamps as an actual date and time would be more interesting. LTTng > stores the real time at the beginning of the trace and stores it in the > metadata, which is later used to convert the timestamps from monotonic time > to real time. If you change the system's real time while tracing, it won't > have any effect on the timestamps of the recorded events. > > Mohamad > > [1] https://linux.die.net/man/3/clock_gettime > [2] http://linuxmogeb.blogspot.ca/2013/10/how-does-clockgettime-work.html > > > On 04/20/2018 11:37 AM, Jérémie Galarneau wrote: > > Hi, > > I'm adding the LTTng mailing list in CC for the benefit of the community > and so people add to or correct my explanation. > > The CTF specification describes the meaning of those fields: > http://diamon.org/ctf/#spec8 > > In your case, this means that the timestamps in the trace are expressed > relative to an arbitrary time set 1524226572435601778 clock ticks after > 00:00:00 on 1 January 1970. > > To take your specific example: > Your clock is running at 1,000,000,000 Hz, meaning the duration of a clock > tick (period) is of 1 ns. > > The event has occurred at > 1524226572435601778 (offset) + 00000003827855360941 (event clock value) > nanoseconds after UNIX Epoch. That leads us to around Friday, April 20, > 2018 1:20:00 PM (GMT). > > Regards, > Jérémie > > > > On 20 April 2018 at 11:06, Sai Tujala <sai.tuj...@mathworks.in> wrote: > >> Hi Jeremie, >> >> >> In metadata file of LTTng trace session, offset from >> Epoch is “1524226572435601778” and time stamp of an event in channel data >> files is starting from “00000003827855360941”. Can you brief about >> relevance between these two time stamps. >> >> >> freq = 1000000000; /* Frequency, in Hz */ >> >> /* clock value offset from Epoch is: offset * (1/freq) */ >> >> offset = 1524226572435601778; >> >> >> >> Thanks & Regards, >> >> T Sai Kiran >> >> > > > > -- > Jérémie Galarneau > EfficiOS Inc. > http://www.efficios.com > > > _______________________________________________ > lttng-dev mailing > listlttng-dev@lists.lttng.orghttps://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev > > > _______________________________________________ > lttng-dev mailing list > lttng-dev@lists.lttng.org > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev > > > -- Jérémie Galarneau EfficiOS Inc. http://www.efficios.com
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