Re: [PATCH V6 4/6] perf, x86: handle multiple records in PEBS buffer

[Peter Zijlstra]

On Fri, Apr 17, 2015 at 08:20:37PM +0200, Andi Kleen wrote:
> On Fri, Apr 17, 2015 at 04:44:07PM +0200, Peter Zijlstra wrote:
> > On Fri, Apr 17, 2015 at 02:19:58PM +0000, Liang, Kan wrote:
> > 
> > > > But that brings us to patch 1 of this series, how is that correct in the face of
> > > > this? There is an arbitrary delay (A->B) added to the period.
> > > > And the Changelog of course never did bother to make that clear.
> 
> That's how perf and other profilers always behaved. The PMI
> is not part of the period. The automatic PEBS reload is not in any way
> different. It's much faster than a PMI, but it's also not zero cost.
> 
> This is not a gap in measurement though -- there is no other code
> running during that time on that CPU. It's simply overhead from the
> measurement mechanism.
> 
> > > 
> > > OK. I will update the changelog for patch 1 as below.
> > > ---
> > > When a fixed period is specified, this patch make perf use the PEBS
> > > auto reload mechanism. This makes normal profiling faster, because
> > > it avoids one costly MSR write in the PMI handler.
> > 
> > > However, the reset value will be loaded by hardware assist. There is 
> > > a little bit delay compared to previous non-auto-reload mechanism.
> > > The delay is arbitrary but very small.
> > 
> > What is very small? And doesn't that mean its significant at exactly the
> > point this patch series is aimed at, namely very short period.
> 
> The assist cost is 400-800 cycles, assuming common cases with everything
> cached. The minimum period the patch currently uses is 10000. In that
> extreme case it can be ~10% if cycles are used.

Thanks, please include all this information.
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