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

* Carl Love <cel@us.ibm.com> wrote:

>
> On Fri, 2009-05-29 at 10:21 +0200, Geert Uytterhoeven wrote:
> > On Fri, 29 May 2009, Ingo Molnar wrote:
> > > * Paul Mackerras <paulus@samba.org> wrote:
> > > > Ingo Molnar writes:
> > > > > * Corey Ashford <cjashfor@linux.vnet.ibm.com> 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 ;-)
> >
> > I never really dived into this stuff, but ISTR there are some 16-bit counters
> > on CELL? Is that correct?
>
> FYI, the counters on CELL are configurable. You can have up to
> eight 16 bit count counters or you can combine counters i and i=1
> (where i=0,2,4,6) into 32 bit counters. This allows you to have
> some 16 and some 32 bit counters at the same time.

If 16-bit counters are exposed then this can be solved like the
PowerPC perfcounters code does it: by not propagating 'early' IRQs
back to the generic layer but continuing it until the real threshold
has been reached.

Ingo


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