linux-kernel - Re: [lttng-dev] [PATCH 09/11] sched: export task

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Date:	Fri, 23 Dec 2011 11:51:28 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frank Rowand <frank.rowand@...sony.com>
Cc:	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Greg KH <greg@...ah.com>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lttng-dev@...ts.lttng.org" <lttng-dev@...ts.lttng.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [lttng-dev] [PATCH 09/11] sched: export task_prio to GPL modules


* Frank Rowand <frank.rowand@...sony.com> wrote:

> On 12/20/11 03:08, Ingo Molnar wrote:
> > 
> > (Cc:-ing Arnaldo on this as well.)
> > 
> > * Mathieu Desnoyers <compudj@...stal.dyndns.org> wrote:
> > 
> 
> < snip >
> 
> > I think your concentration on ABIs is missing a very fundamental 
> > property of instrumentation:
> > 
> >   the life-time and persistence of instrumentation data is 
> >   typically very short ('days' is already an exception - typical 
> >   is minutes, at most hours), and for that reason we havent been 
> >   getting much pressure from users to maintain a perf.data ABI - 
> >   but we are doing it nevertheless.
> > 
> > Instrumentation is fundamentally about the 'here and now' and so 
> > it fundamentally differs from things like backup formats and 
> > database formats. An ABI does not hurt and we are maintaining 
> > it, but you are overrating its importance significantly.
> 
> Just to provide visibility to a different use case...
> 
> The life time of my data is typically weeks, months, or years 
> (though I am not likely to re-process year old raw data).

I'm not saying that it's absolutely never done: for example 
monitoring/logging on a production box and evaluating events 
only once per month would certainly qualify.

I just say that the overwhelming majority of usecases utilize 
traces on a short time-span and that we must keep the common 
usecase in mind when supporting not so common usecases.

It's the same deal as with -rt: compared to the 'normal' usage 
of Linux -rt is somewhat of a special case - yet it's still 
something very much worth doing, as long as the main usecase is 
always kept in mind.

Thanks,

	Ingo
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