On Thu, May 20, 2021 at 5:41 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > Len, > > On Thu, May 20 2021 at 17:22, Len Brown wrote: > > On Thu, May 20, 2021 at 4:54 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > >> > AMX is analogous to the multiplier used by AVX-512. > >> > The architectural state must exist on every CPU, including HT siblings. > >> > Today, the HT siblings share the same execution unit, > >> > and I have no reason to expect that will change. > >> > >> I'm well aware that HT siblings share the same execution unit for > >> AVX. > >> > >> Though AMX is if I remember the discussions two years ago correctly > >> shared by more than the HT siblings which makes things worse. > > > > I regret that we were unable to get together in the last year to have > > an updated discussion. I think if we had, then we would have saved > > a lot of mis-understanding and a lot of email! > > > > So let me emphasize here: > > > > There is one TMUL execution unit per core. > > It is shared by the HT siblings within that core. > > > > So the comparison to the AVX-512 multiplier is a good one. > > Fine, but that does not at all change the facts that: > > 1) It's shared between logical CPUs > > 2) It has effects on power/thermal and therefore effects which reach > outside of the core scope FWIW, this is true of *every* instruction in the CPU. Indeed, even when the CPU is executing *no* instructions at all, the C-state chosen by that CPU has power/thermal impacts on its peers. Granted, high performance instructions such as AVX-512 and TMUL are the most extreme case. > 3) Your approach of making it unconditionally available via the > proposed #NM prevents the OS and subsequently the system admin / > system designer to implement fine grained control over that > resource. > > And no, an opt-in approach by providing a non-mandatory > preallocation prctl does not solve that problem. I'm perfectly fine with making the explicit allocation (aka opt-in) mandatory, and enforcing it. Len Brown, Intel Open Source Technology Center
