linux-kernel - LTTng and SystemTAP (Everyone who is scared to read this huge thread, skip to here)

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Date:	Mon, 18 Sep 2006 01:03:10 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Karim Yaghmour <karim@...rsys.com>, Theodore Tso <tytso@....edu>,
	Nicholas Miell <nmiell@...cast.net>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jes Sorensen <jes@....com>,
	Andrew Morton <akpm@...l.org>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>,
	"Martin J. Bligh" <mbligh@...igh.org>
Subject: LTTng and SystemTAP (Everyone who is scared to read this huge thread, skip to here)

* Ingo Molnar (mingo@...e.hu) wrote:
> so regarding the big picture we are largely on the same page in essence 
> i think - sub-issues non-withstanding :-) As long as LTT comes with a 
> facility that allows the painless moving of a static LTT markup to a 
> SystemTap script, that would come quite a bit closer to being acceptable 
> for upstream acceptance in my opinion.
> 
> The curious bit is: why doesnt LTT integrate SystemTap yet? Is it the 
> performance aspect?

Yes, for our needs, the performance impact of SystemTAP is too high. We are
totally open to integrate data coming from SystemTAP to our traces. Correct me
if I am wrong, but I think their project does an extensive use of strings in the
buffers. This is one, non compact, sub-optimal type, but it can do the job for
low rate events. I also makes classification and identification of the
information rather less straightforward. Plus, running a string format code in a
critical code path does not give the kind of performance I am looking for.

> Some of the extensive hooking you do in LTT could be 
> aleviated to a great degree if you used dynamic probes. For example the 
> syscall entry hackery in LTT looks truly scary.

Yes, agreed. The last time I checked, I thought about moving this tracing code
to the syscall_trace_entry/exit (used for security hooks and ptrace if I
remember well). I just didn't have the time to do it yet.

> I cannot understand that 
> someone who does tracing doesnt see the fundamental strength of 
> SystemTap - i think that in part must have lead to my mistake of 
> assuming that you opposed SystemTap.
> 

Can you find a way to instrument it dynamically without the breakpoint cost ?
System calls are a highly critical path both in a system and for tracing.

Mathieu


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