Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

Ingo Molnar
2006-09-16T08:31:16Z

* Roman Zippel <zippel@linux-m68k.org> wrote:

> > > > > > - a marker for dynamic tracing has lower performance impact
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > > > > than a static tracepoint, on systems that are not being
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > > > > traced. (but which have the tracing infrastructure enabled
^^^^^^
> > > > > > otherwise)
> > > > >
> > > > > Anyone using static tracing intents to use, which makes this point
> > > > > moot.
> > > >
> > > > that's not at all true, on multiple grounds:
> > > >
> > > > Firstly, many people use distro kernels. A Linux distribution
> > > > typically wants to offer as few kernel rpms as possible (one per
> > > > arch to be precise), but it also wants to offer as many features
> > > > as possible. So if there was a static tracer in there, a distro
> > > > would enable it - but 99.9% of the users would never use it - still
> > > > they would see the overhead. Hence the user would have it enabled,
> > > > but does not intend to use it - which contradicts your statement.
> > >
> > > So if dynamic tracing is available use it, as distributions
> > > already do. OTOH the barrier to use static tracing is drastically
> > > different whether the user has to deal with external patches or
> > > whether it's a simple kernel option. Again, static tracing doesn't
> > > exclude the possibility of dynamic tracing, that's something you
> > > constantly omit and thus make it sound like both options were
> > > mutually exlusive.
> >
> > how does this reply to my point that: "a marker for dynamic tracing has
> > lower performance impact than a static tracepoint, on systems that are
> > not being traced", which point you claimed moot?
>
> Because it's pretty much an implementation issue. [...]

No, that's my point, it's not an "implementational issue" of static
tracers, the overhead of markups for static tracers is:

_inherent to their concept of being compile-time and static_

ok?

> [...] The point is about adding markers at all, it's about the choice
> being able to use static tracers in the first place. [...]

your characterization of "the point" is at odds with the specific point
that we are discussing - see the underlined sentence above, right at the
top of the quotes:

> > > > > > - a marker for dynamic tracing has lower performance impact
> > > > > > than a static tracepoint, on systems that are not being
> > > > > > traced. (but which have the tracing infrastructure enabled

Please either concede the point or dispute it, before shifting to new
grounds. Thanks,

Ingo