Re: [Jack-Devel] AMD Bulldozer CPUs, shared FPU and Intel Hyper-threading

On Thu, 2013-02-21 at 15:25 +0200, Nedko Arnaudov wrote:
> Clemens Ladisch <[hidden]> writes:
> 
> > Chris Caudle wrote:
> >> The issue with bulldozer (and piledriver) is that two integer cores share
> >> a single floating point core, so the scheduler thinks it can schedule two
> >> cores, but depending on whether the processes being scheduled are mostly
> >> integer or mostly floating point, it might work well or there might be
> >> contention for the single FP core.
> >
> > AMD says (in the optimization manual):
> > | The AMD Family 15h processor floating point unit (FPU) was designed to
> > | provide four times the raw FADD and FMUL bandwidth as the original AMD
> > | Opteron and Athlon 64 processors.  It achieves this by means of two
> > | 128-bit fused multiply-accumulate (FMAC) units which are supported by
> > | a 128-bit high-bandwidth load-store system. [...]
> > | The FPU can receive up to four ops per cycle. These ops can only be
> > | from one thread, but the thread may change every cycle. Likewise the
> > | FPU is four wide, capable of issue, execution and completion of four
> > | ops each cycle. Once received by the FPU, ops from multiple threads
> > | can be executed.
> >
> > So if you're running two FP-heavy threads that happen to interfere with
> > each other in the most negative way, the theoretical performance will
> > only be double that of earlier CPUs ...
> >
> > In practice, the FPU sharing matters only if you use 256-bit AVX
> > instructions, because those must be executed in two steps.
> 
> It is expected that performance of the newer CPU family is better. The
> point is the possibility of xruns happening occasionally at 50% DSP
> load.

Has this been tested? 

It is interesting, although I'm not upgrading for nearly 2 years... but
I do recommend systems to people.. currently I have a core i7 desktop
with hyper-threading turned off for this.. I recommend core i5 ... but
this machine will become my home server in 2 years so it's okay for
that.. 

If AMD can use all 8 cores and not suffer in due to shared fpu (i.e. not
using 256 bit AVX from how I read this) then the AMD becomes better
value for money .. 






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