dmcq (dmcq.delete@this.fano.co.uk) on October 31, 2015 8:19 am wrote:
> bakaneko (nyan.delete@this.hyan.wan) on October 31, 2015 7:28 am wrote:
> > dmcq (dmcq.delete@this.fano.co.uk) on October 30, 2015 5:12 am wrote:
> > > lurker (lurker9000.delete@this.realemail.mail) on October 30, 2015 2:39 am wrote:
> > > > > First of all - welcome to RWT, glad to hear your perspective.
> > > >
> > > > Thanks.
> > > >
> > > > > My guess is that everything you say is true...
> > > >
> > > > Eh, I just thought I'd post what I heard from a guy who supposedly worked on Zen.
> > > >
> > > > > and that AMD isn't intending to hit the HPC
> > > > > market. They have 128b vectors (since that's all ARM supports), which simply isn't wide
> > > > > enough to be competitive with Skylake. So giving up on a third AGU makes sense. The third
> > > > > AGU is probably most helpful for HPC (where they cannot compete anyway) and isn't a particularly
> > > > > small unit in terms of design complexity and impact on the load/store buffer.
> > > > >
> > > > > David
> > > >
> > > > 128bit FP pipes seem optimal for most desktop and server software. HPC is pretty
> > > > much the only place where latest instructions are used and even if zen was competitive
> > > > here I don't think anyone would want to switch from Intel.
> > > > Personally I just hope the lack of 3rd AGU won't cause problems in SMT. I don't think
> > > > normal workloads have that many operations that access memory, but SMT aims to maximize
> > > > utilization of all available resources and only 2 AGUs might be a problem there.
> > >
> > > I think 256 bits would be better as you can do four double precision operations at once and that is quite
> > > common. On the other hand with four SIMD units instead one could merge two operations to give an effective
> > > two by 256 bit units except for some special operations. For anything larger they'd probably be better
> > > off relying on GPUs I think if they can get the coherence and message passing working well. I can see
> > > how to save larger register sets without impacting interrupt handling too badly but it seems a lot of
> > > work when ARM is probably hoping to move 64 bit ARM into the embedded processor market.
> >
> > Except nobody sane would work on large amounts of
> > doubles for most normal applications. It's really
> > only useful for HPC, and there some other things
> > probably matter even more, as floating point can be
> > the wrong hammer.
>
> Well I know games often just use floats in the GPUs an some AI people say 8-bit integers are enough
> for any useful AI problem - but it is amazing how fast a sequence of float operations can start to give
> obviously wrong results. If one wants half a chance of something approximating a reasonable result and
> aren't an expert at error analysis there's nothing to beat just doing the work using doubles.

While it sounds worthwile, it is actually wrong.
More bits don't save you from errors; in some cases
they will give you worse results.

Floats is one of these topics where you can't go with
(pretty naive) hunches. You need to understand the
material properly.