linux-kernel - Re: [patch 0/3] [Announcement] Performance Counters for Linux

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Message-ID: <20081205084233.GE2030@elte.hu>
Date:	Fri, 5 Dec 2008 09:42:33 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	a.p.zijlstra@...llo.nl, paulus@...ba.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	akpm@...ux-foundation.org, eranian@...glemail.com,
	dada1@...mosbay.com, robert.richter@....com, arjan@...radead.org,
	hpa@...or.com, rostedt@...dmis.org
Subject: Re: [patch 0/3] [Announcement] Performance Counters for Linux

* David Miller <davem@...emloft.net> wrote:

> From: Ingo Molnar <mingo@...e.hu>
> Date: Fri, 5 Dec 2008 09:24:31 +0100
> 
> > Right now we begun with the most trivial ones:
> > 
> >   enum perf_record_type {
> >           PERF_RECORD_SIMPLE,
> >           PERF_RECORD_IRQ,
> >   };
> > 
> > ... but it would be natural to do a PERF_RECORD_GP_REGISTERS as well. 
> > Perhaps even a PERF_RECORD_STACKTRACE using the sysprof facilities, to do 
> > a hierarchic multi-dimension profile that sysprof does so nicely.
> 
> Maybe even add something like PERF_RECORD_THE_MOON...
> 
> see how rediculious this is?

Note that more notification record types is actually where latest 
hardware is going: for example in Nehalem there's a PEBS notification 
record type that has cachemiss latency included in the record. I.e. we 
can get profiles with _cachemiss latency_ included (as measured from 
issuing the instruction to completion).

You cannot get that information out of any 'stop the task' interface ...

Stopping a task is way too intrusive, i dont know why you keep harping on 
it. Listen to the scheduler guys: it's a non-starter.

> It's not your business in the kernel to decide what things are useful.  
> The monitor can stop the task and inspect whatever it wants with 
> _existing_ facilities.  We need none of this stuff.

You try to ridicule our efforts, while you have not answered our 
technical arguments in substance.

Please let me repeat: it's a _fundamental_ thesis of performance 
instrumentation to not disturb the monitored context. Your insistence on 
_stopping_ the monitored task breaks that fundamental axiom!

Stopping a task destroys the characteristics of many, many workloads. To 
get a reasonable histogram out of a system a highlevel event count of 
thousands a second is desired (but hundreds of them are a minimum, to get 
any reasonable coverage).

But injecting even hundreds of artificialy task-stoppages will destroy 
the true behavior of many reference workloads we care about in Linux!

Stopping the task is a fundamental and obvious design failure of perfmon.

	Ingo
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