Re: [perfmon2] comments on Performance Counters for Linux (PCL)

[Ingo Molnar]

* Paul Mackerras <paulus@xxxxxxxxx> wrote:

> Ingo Molnar writes:
> 
> > * Corey Ashford <cjashfor@xxxxxxxxxxxxxxxxxx> wrote:
> > >> So you're suggesting to artificually strech periods by say 
> > >> composing a single overflow from smaller ones, ignoring the 
> > >> intermediate overflow events?
> > >>
> > >> That sounds doable, again, patch welcome.
> > >
> > > I definitely agree with Stephane's point on this one.  I had 
> > > assumed that long irq_periods (longer than the width of the 
> > > counter) would be synthesized as you suggest.  If this is not the 
> > > case, PCL should be changed so that it does, -or- at a minimum, 
> > > the user should get an error back stating that the period is too 
> > > long for the hardware counter.
> > 
> > this looks somewhat academic - at least on x86, even the fastest 
> > events (say cycles) with a 32 bit overflow means one event per 
> > second on 4GB. That's not a significant event count in practice. 
> > What's the minimum width we are talking about on Power?
> 
> 32 bits, but since the top bit is effectively a level-sensitive 
> interrupt request, the maximum period in hardware is 2^31 counts.
> 
> However, I already support 64-bit interrupt periods (well, 63-bit 
> actually) on powerpc by only calling perf_counter_overflow() when 
> counter->hw.period_left becomes <= 0, and arranging to set the 
> hardware counter to 0 if counter->hw.period_left is >= 0x80000000. 
> It's a tiny amount of code to handle it, really.

No argument about that - just wanted to know whether there's any 
real practical effect beyond the nitpicking factor ;-)

	Ingo
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