linux-kernel - Re: [PATCH 2/4] ftrace - add function

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Date:	Thu, 10 Dec 2009 13:50:44 -0500
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tim Bird <tim.bird@...sony.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] ftrace - add function_duration tracer

Hi -


On Thu, Dec 10, 2009 at 07:35:08PM +0100, Ingo Molnar wrote:
> [...]
> target_set.stp is not really adequate. Have you actually _tried_ to use 
> it on something real like hackbench, which runs thousands (or tens of 
> thousands) of tasks? You'll soon find that associative arrays are not 
> really adequate for that ... [...]

A few thousand entries in a hash table is really not that big a deal.


> > > Also, i dont think stap supports proper separation of per workload 
> > > measurements either. I.e. can you write a script that will work 
> > > properly even if multiple monitoring tools are running, each trying 
> > > to measure latencies?
> > 
> > Sure, always has.  You can run many scripts concurrently, each with 
> > its own internal state.  (Overheads accumulate, sadly & naturally.)
> 
> To measure latencies you need two probes, a start and a stop one. How do 
> you define a local variable that is visible to those two probes? You 
> have to create a global variable - but that will/can clash with other 
> instances.

You misunderstand systemtap "global" values.  They are global to that
particular execution of that particular script.  They are not shared
between scripts that may be concurrently running.


> ( Also, you dont offer per application channels/state from the same 
>   script. Each app has to define their own probes, duplicating the 
>   script and increasing probe chaining overhead. )

Please elaborate what you mean.


> > > Also, i personally find built-in kernel functionality more trustable 
> > > than dynamically built stap kernel modules that get inserted.
> > 
> > I understand.  In the absence of a suitable bytecode engine in the 
> > kernel, this was the only practical way to do everything we needed.
> 
> You seem to be under the mistaken assumption that your course of action 
> with SystemTap is somehow limited by what is available (or not) in the 
> upstream kernel. In reality you can implement anything you want [...]

The message we have received time, after time, after time was
stronger: that a suitable interpreter was not going to be welcome in
tree.  If this is relaxed (and perhaps even if not), we may prototype
such a thing in the new year.


- FChE
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