Messages in this thread | | | Subject | Re: [PATCH 2/5] perf, x86: Enable PDIR precise instruction profiling on IvyBridge | From | Peter Zijlstra <> | Date | Mon, 02 Jul 2012 21:18:53 +0200 |
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On Mon, 2012-07-02 at 11:43 -0700, Andi Kleen wrote: > From: Andi Kleen <ak@linux.intel.com> > > Even with precise profiling Intel CPUs have a "skid". The sample > triggers a few cycles later than the instruction, so in some > cases there can be systematic errors where expensive instructions never > show up in the profile log. > > Sandy Bridge added a new PDIR instruction retired event that randomizes > the sampling slightly. This corrects for systematic errors, so that > you should in most cases see the correct instruction getting profile hits. > > Unfortunately the SandyBridge version could only work with a otherwise > quiescent CPU and was difficult to use. But now on IvyBridge this > restriction is gone and can be more widely used. > > This only works for retired instructions. > > I enabled it -- somewhat arbitarily -- for two 'p's or more. > > To use it > > perf record -e instructions:pp ... > > This provides a more precise alternative to the usual cycles:pp, > however it will not account for expensive instructions.
Already told you no.
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