On Thu, Jan 31, 2002 at 09:32:48PM +0100, Gérard Roudier wrote: > > > On Thu, 31 Jan 2002, Jason Evans wrote: > > > On Wed, Jan 30, 2002 at 11:14:48PM +0100, Gérard Roudier wrote: > > > > > > Linux can be fixed, but the useless writes of the existing Athlons from > > > the very fast cache to the relatively very slow memory cannot. And all > > > Athlon users may well pay this penalty under any OS... unless we want to > > > disable caching. :) > > > > Have you done benchmarks to show that the speculative writes are useless > > often enough to cause enough memory bus contention that overall performance > > is degraded, despite the speedups when the speculative writes are valid? > > I haven't done any benchmark of this sort, neither intend to do any since > I haven't time for that. But I wrote in my email that my 2 Athlon systems > worked fine and fast, just to indicate that for normal use I didn't see > any performance problem at all. > > > I > > suspect that AMD in fact performed such tests; otherwise they wouldn't have > > gone to the trouble of implementing speculative writes. > > The Athlon rewriting same value to cacheable memory under the knees of > programmers looks a severe issue to me if it is true. Not only AGP memory > can be affected. What about SMP, MMIO (if some cacheable mapping exists), > etc...?
I am not familiar with the acronym MMIO is so I can't comment on that. In general though, having memoryspace used for memory-mapped I/O devices (including AGP) marked as cacheable is a bad idea unless you are very careful and know exactly what you are doing. For SMP it shouldn't be any problem. Multi-CPU systems normally run some cache-coherence protocol between themselves to make sure that things like this is not a problem. > > In my opinion, OSes having some cacheable mapping to AGP memory is not the > real problem. Just it has revealed the AMD issue. It might be argued that there should be some cache-coherence protocol between the CPU and the AGP device. Not knowing how AGP is specified I don't know if this interaction between the CPU and AGP is a bug or just working as specified. I suspect it is the latter though. -- <Insert your favourite quote here.> Erik Trulsson [EMAIL PROTECTED] To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
