Subject: Re: [Perfctr-devel] Re: Enabling RDPMC in user space by default
From: Lee Revell <rlrevell@joe-job.com>
To: John Reiser <jreiser@BitWagon.com>
Cc: Andi Kleen <ak@suse.de>, Stephane Eranian <eranian@hpl.hp.com>,
       Ray Bryant <raybry@mpdtxmail.amd.com>, discuss@x86-64.org,
       linux-kernel@vger.kernel.org, perfctr-devel@lists.sourceforge.net
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	 <20051129181344.GN19515@wotan.suse.de>  <438C9E1B.8040204@BitWagon.com>
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Date: Tue, 29 Nov 2005 14:05:43 -0500
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On Tue, 2005-11-29 at 10:29 -0800, John Reiser wrote:
> Andi Kleen wrote:
> > I think it's also a useful convention - RDTSC is becomming more and more
> > useless and you cannot expect user applications who just want to
> > measure some cycles to rely on ever changing instable or non existing
> > performance counter APIs.
> 
> Users are even more unhappy with ever-changing ABIs -- such as the
> kernel taking away RDTSC.
> 
> RDTSC+perfctr [Pettersson] still is the fastest way for user-mode code
> to count something that is highly correlated with both "billable"
> CPU time and "code quality" for a fixed task.  With a little care
> RDTSC is close enough to monotonic that I find it very useful.
> Please don't take away user-mode RDTSC.
> 

The kernel didn't take this away, the hardware vendors did.  For years
although it was risky on paper it was perfectly usable as a cheap high
res timer.  I agree that it's unfortunate.

Lee

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