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

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Date:	Sat, 16 Sep 2006 10:23:10 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, karim@...rsys.com,
	Andrew Morton <akpm@...l.org>,
	Paul Mundt <lethal@...ux-sh.org>, Jes Sorensen <jes@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Tom Zanussi <zanussi@...ibm.com>, ltt-dev@...fik.org,
	Michel Dagenais <michel.dagenais@...ymtl.ca>
Subject: Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108


* Roman Zippel <zippel@...ux-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
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