Re: [perfmon2] comments on Performance Counters for Linux (PCL)
From: Ingo Molnar
Date: Fri May 29 2009 - 03:19:50 EST
* 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 ;-)
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