Re: [patch] Performance Counters for Linux, v3

Peter Zijlstra
2008-12-12T09:01:20Z

On Fri, 2008-12-12 at 09:51 +0100, Peter Zijlstra wrote:
> 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?

http://icl.cs.utk.edu/projects/papi/files/html_man3/papi_presets.html

Has a lot of cache events.

And I can see the use of a set without the L[123] in there, which would
signify either all or the lack of more specific knowledge. Like with
PAPI its perfectly fine to not support these common events on a
particular hardware platform.