Message-ID: <438C9E1B.8040204@BitWagon.com>
Date: Tue, 29 Nov 2005 10:29:47 -0800
From: John Reiser <jreiser@BitWagon.com>
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To: Andi Kleen <ak@suse.de>
CC: 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
Subject: Re: [Perfctr-devel] Re: Enabling RDPMC in user space by default
References: <20051129151515.GG19515@wotan.suse.de> <200511291056.32455.raybry@mpdtxmail.amd.com> <20051129180903.GB6611@frankl.hpl.hp.com> <20051129181344.GN19515@wotan.suse.de>
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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.

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