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

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Message-ID: <20060915220345.GC12789@elte.hu>
Date:	Sat, 16 Sep 2006 00:03:45 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Jose R. Santos" <jrs@...ibm.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Andrew Morton <akpm@...l.org>, tglx@...utronix.de,
	karim@...rsys.com, Paul Mundt <lethal@...ux-sh.org>,
	Jes Sorensen <jes@....com>, 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

* Jose R. Santos <jrs@...ibm.com> wrote:

> [...]  While it is true that static probes will provide less overhead 
> compared to dynamic probes, [...]

that is not true at all. Yes, an INT3 based kprobe might be expensive if 
+0.5 usecs per tracepoint (on a 1GHz CPU) is an issue to you - but that 
is "only" an implementation detail, not a conceptual property. 
Especially considering that help (djprobes) is on the way. And in the 
future, as more and more code gets generated (and regenerated) on the 
fly, dynamic probes will be _faster_ than static probes - plainly 
because they adapt better to the environment they plug into.

so there's basically nothing to balance. My point is that dynamic probes 
have won or will win on every front, and we shouldnt tie us down with 
static tracers. 5 years ago with no kprobes, had someone submitted a 
clean static tracer patchset, we could probably not have resisted it (i 
though probably would have resisted it on the grounds of maintainance 
overhead) and would have added it because tracing makes sense in 
general. But today there's just no reason to add static tracers anymore.

NOTE: i still accept the temporary (or non-temporary) introduction of 
static markers, to help dynamic tracing. But my expectation is that 
these markers will be less intrusive than static tracepoints, and a lot 
more flexible.

	Ingo
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