Re: [RFC][PATCH 5/5] perfcounter: Add support for kernel hardwarebreakpoints

[Frederic Weisbecker]

On Tue, Jul 28, 2009 at 09:24:37AM +0200, Peter Zijlstra wrote:
> On Tue, 2009-07-28 at 03:03 +0200, Frederic Weisbecker wrote:
> > > Thirdly, we can multiplex perf counters beyond their hardware maximum,
> > > something you simply cannot do for a debug interface.
> > 
> > 
> > Again, I'm stuck in what you mean by multiplexing here :-)
> 
> If you'd create say 16 breakpoint counters, we'd RR them over the 4
> available hardware breakpoints (or less when others are taken by someone
> else).
> 
> Since its all statistics anyway, we can simply scale the event counts up
> by the time-share they received.
>

Aah, ok I understand now.
But I fear it may kill the accuracy of the breakpoints statistics.
It's fine for a theoretical linear rate of breakpoint events.
But what happens if we are profiling something much more unstable
with a rain of hits in a small window of memory between large timeframes?
If we have a breakpoint inside this window of memory and this breakpoint
is not plugged, waiting for its turn in the RR, we loose this rain of events.

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