linux-kernel - Re: [perfmon] Re: [perfmon2] perfmon2 merge news

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Message-ID: <20071114142629.GE17145@one.firstfloor.org>
Date:	Wed, 14 Nov 2007 15:26:29 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Stephane Eranian <eranian@....hp.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, Philip Mucci <mucci@...utk.edu>,
	William Cohen <wcohen@...hat.com>,
	Robert Richter <robert.richter@....com>,
	linux-kernel@...r.kernel.org, Perfmon <perfmon@...ali.hpl.hp.com>,
	perfmon2-devel@...ts.sourceforge.net,
	OSPAT devel <ospat-devel@...utk.edu>,
	papi list <ptools-perfapi@...utk.edu>
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news

On Wed, Nov 14, 2007 at 06:13:42AM -0800, Stephane Eranian wrote:
> > At least for x86 and I suspect some 1other architectures we don't
> > initially need a syscall at all for this. There is an instruction
> > RDPMC who can read a performance counter just fine. It is also much
> > faster and generally preferable for the case where a process measures
> > events about itself. In fact it is essential for one of the use cases
> > I would like to see perfmon used (replacement of RDTSC for cycle
> > counting) 
> > 
> 
> This only works when counting (not sampling) and only for self-monitoring.

It works for global monitoring too.

> 
> > Later a syscall might be needed with event multiplexing, but that seems
> > more like a far away non essential feature.
> > 
> On a machine with only two generic counters such as MIPS or Intel Core 2 Duo,
> multiplexing offers some advantages. If NMI watchdog is enabled, then you drop
> to one generic counter on on Core 2.

NMI watchdog is off by default now.

Yes longer term we might need multiplexing, but definitely not as first step.

-Andi
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