Messages in this thread |  | | | Subject | Re: [Perfctr-devel] Re: Enabling RDPMC in user space by default | | From | Nicholas Miell <> | | Date | Tue, 29 Nov 2005 15:29:26 -0800 |
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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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