Re: [perfmon2] IV.3 - AMD IBS

[Maynard Johnson]

Rob Fowler <rjf@xxxxxxxxx> wrote on 06/22/2009 09:08:34 AM:

>
>
> Ingo Molnar wrote:
> >> 3/ AMD IBS
> >>
> >> How is AMD IBS going to be implemented?
> >>
> >> IBS has two separate sets of registers. One to capture fetch
> >> related data and another one to capture instruction execution
> >> data. For each, there is one config register but multiple data
> >> registers. In each mode, there is a specific sampling period and
> >> IBS can interrupt.
> >>
> >> It looks like you could define two pseudo events or event types
> >> and then define a new record_format and read_format. That formats
> >> would only be valid for an IBS event.
> >>
> >> Is that how you intend to support IBS?
> >
> > That is indeed one of the ways we thought of, not really nice, but
> > then, IBS is really weird, what were those AMD engineers thinking
> > :-)
>
> Actually, IBS has roots in DEC's "ProfileMe" for Alpha EV67 and later
> processors.   Those of us who used it there found it to be an extremely
> powerful, low-overhead mechanism for directly collecting information
about
> how well the micro-architecture is performing.  In particular, it can
tell
> you, not only which instructions take a long time to traverse the pipe,
but
> it also tells you which instructions delay other instructions and byhow
much.
> This is extremely valuable if you are either working on instruction
scheduling
> in a compiler, or are modifying a program to give the compiler the
opportunity
> to do a good job.
>
> A core group of engineers who worked on Alpha went on to AMD.
>
> An unfortunate problem with IBS on AMD is that good support isn't
> common in the "mainstream"
> open source community.
>
> Code Analyst from  AMD, also involving ex-DEC engineers, is
> the only place where it is supported at all decently throughout the
> tool chain.
> Last  time I looked, there was a tweaked version of oprofile underlying
CA.
> I haven't checked to see whether the tweaks have migrated back into
> the oprofile trunk.
Yes, IBS on AMD is now supported upstream in mainline, contributed by
Suravee Suthikulpanit.

-Maynard
>
>
> >
> > The point is - weird hardware gets expressed as a ... weird feature
> > under perfcounters too. Not all hardware weirdnesses can be
> > engineered away.
> >
> >
>
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> --
> Robert J. Fowler
> Chief Domain Scientist, HPC
> Renaissance Computing Institute
> The University of North Carolina at Chapel Hill
> 100 Europa Dr, Suite 540
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