Re: [patch] Performance Counters for Linux, v3

Peter Zijlstra
2008-12-12T08:51:41Z

On Fri, 2008-12-12 at 09:35 +0100, stephane eranian wrote:
> Peter,
>
> On Fri, Dec 12, 2008 at 9:25 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> >> > + /*
> >> > + * Common hardware events, generalized by the kernel:
> >> > + */
> >> > + PERF_COUNT_CYCLES = 0,
> >> > + PERF_COUNT_INSTRUCTIONS = 1,
> >> > + PERF_COUNT_CACHE_REFERENCES = 2,
> >> > + PERF_COUNT_CACHE_MISSES = 3,
> >> > + PERF_COUNT_BRANCH_INSTRUCTIONS = 4,
> >> > + PERF_COUNT_BRANCH_MISSES = 5,
> >>
> >> Many machines do not support these counts. For example, Niagara T1 does
> >> not have a CYCLES count. And good luck if you think you can easily come
> >> up with something meaningful for the various kind of CACHE_MISSES on the
> >> Pentium 4. Also, the Pentium D has various flavors of retired instruction
> >> count with slightly different semantics. This kind of abstraction should
> >> be done in userspace.
> >
> > I'll argue to disagree, sure such events might not be supported by any
> > particular hardware implementation - but the fact that PAPI gives a list
> > of 'common' events means that they are, well, common. So unifying them
> > between those archs that do implement them seems like a sane choice, no?
> >
> > For those archs that do not support it, it will just fail to open. No
> > harm done.
> >
> > The proposal allows for you to specify raw hardware events, so you can
> > just totally ignore this part of the abstraction.
> >
> I believe the cache related events do not belong in here. There is no definition
> for them. You don't know what cache miss level, what kind of access. You cannot
> do this even on Intel Core processors.

I might agree with that, perhaps we should model this to the common list
PAPI specifies?