On Wed, Feb 3, 2010 at 3:19 PM, Peter Zijlstra <pet...@infradead.org> wrote: > On Wed, 2010-02-03 at 15:07 +0100, Stephane Eranian wrote: >> >> The only improvement that PEBS provides is that you get an IP and the >> >> machine state at retirement of an instruction that caused the event to >> >> increment. Thus, the IP points to the next dynamic instruction. The >> >> instruction >> >> is not the one that cause the P-th occurence of the event, if you set the >> >> period to P. It is at P+N, where N cannot be predicted and varies >> >> depending >> >> on the event and executed code. This introduces some bias in the samples.. >> > >> > I'm not sure I follow, it records the next event after overflow, doesn't >> > that make it P+1? >> > >> That is not what I wrote. I did not say if records at P+1. I said it records >> at P+N, where N varies from sample to sample and cannot be predicted. >> N is expressed in the unit of the sampling event. > > OK, so I'm confused. > > The manual says it arms the PEBS assist on overflow, and the PEBS thing > will then record the next event. Which to me reads like P+1. > you are assuming arming is instantaneous.
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