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    SubjectRe: [Perfctr-devel] Re: Enabling RDPMC in user space by default
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    On Wed, 2005-11-30 at 00:17 +0100, Andi Kleen wrote:
    > On Tue, Nov 29, 2005 at 03:02:18PM -0800, Nicholas Miell wrote:
    > > On Tue, 2005-11-29 at 23:43 +0100, Andi Kleen wrote:
    > > > To give an bad analogy RDTSC usage in the last years is
    > > > like explicit spinning wait loops for delays in the earlier
    > > > times. They tended to work on some subset of computers,
    > > > but were always bad and caused problems and people eventually learned
    > > > it was better to use operating system services for this.
    > >
    > > And you are now suggesting people should use RDPMC instead of OS
    > > services?
    >
    > For any kind of timers they should use the OS service
    > (gettimeofday/clock_gettime). The OS will go to extraordinary
    > means to make it as fast as possible, but when it's slow
    > then because it's not possible to do it faster accurately
    > (that's the case right now modulo one possible optimization)
    >
    > For cycle counting where they previously used RDTSC they should
    > use RDPMC 0 now.

    Well, if that's all you want them to use RDPMC 0 for, why not just make
    PMCs programmable from userspace?

    > > That chart contains incompatible variations for pre-B, B, and C revision
    > > processors and (among other strange things) includes instructions for
    > > the monitoring of segment register loads to the HS register.
    > >
    > > Everything is telling me that this is not something AMD intends to keep
    > > stable and it isn't even something they're interested in documenting
    > > very well at all.
    >
    > There are obscure performance counters and then there are basic
    > fundamental performance counters. That particular counter hasn't
    > changed since the K7 days (and K6 didn't have performance counters)

    Sounds like the gamblers fallacy to me, but I'll take your word for it
    for now.

    --
    Nicholas Miell <nmiell@comcast.net>

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