TRACE confusion

Johannes Schlatow schlatow at ...238...
Thu Jul 21 12:52:30 CEST 2016

Hi Josef,

thanks for your explanation.

On Thu, 21 Jul 2016 12:16:28 +0200
Josef Söntgen <josef.soentgen at ...1...> wrote:
> Hello Johannes,
> * Johannes Schlatow <schlatow at ...238...> [2016-07-20 16:49:12 +0200]:
> > The documentation suggests that the trace buffer is assigned to a
> > thread. On the other hand, the trace subject is associated with a
> > session label. What I conclude from this is that a thread be
> > identified by multiple trace subjects but can only be assigned a
> > single trace policy and trace buffer, which makes the interface quite
> > confusing.  
> A Trace::Subject *may* contain a attached Trace::Buffer and at some
> point *did* contain a Trace::Source. To enable tracing the Subject
> must have a valid Trace::Policy that in return is used by the Source
> to fill the Trace::Buffer. A Subject can only be part of one Session.
> Subjects that belong to another Session have the state FOREIGN (see
> [1]).
> [1] repos/base/include/base/trace/types.h
> > I also had a glance at the implementation and noticed that the
> > Trace::Subject has its own buffer, which is forwarded to the
> > Trace::Source. If I understand this correctly, the Trace::Source is
> > coupled to a thread. I therefore assume that I can only trace one
> > trace subject (associated with the same thread) at a time. Is this
> > correct?  
> You can trace as many subjects as your resources permit within a single
> Trace::Session. The Subject outlives the Source (that is why it forwards
> the Buffer to the Source) so one can process the content even after the
> Source has vanished. As you have noticed a Trace::Source is coupled to
> Thread and so is a Subject to its Source. That is a Subject does not get
> reused. After tracing has been enabled for a given Subject it will “live”
> and consum memory as long as it is not freed by the Session, even when
> its Source has vanished. New Subjects will be created in a lazy fashion
> whenever Trace::Session::subjects() is called (see [2]) and there are
> new Sources. Untraced Subjects will vanishes when its Source vanishes.
> [2] repos/base/src/core/
> So from a user perspective or rather from the point of view of a
> Trace-monitor (the component managing the Trace::Session) all you care
> about is a Trace::Subject. To fill the Trace::Buffer, i.e. create
> entries, you care about the Source which is a Thread. That is why the
> methods for creating entries is part of the Thread API.

Okay. I think I got confused by the session label. For a moment I assumed these corresponded to the service provided by the traced component but it's actually the label of the component's CPU session, which makes much more sense now ;-)

Anyway, I have a much better understanding of the implementation now.


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