Re: Scaling noise

[Nick Piggin]

Anton Blanchard wrote:

> > > > I've frequently tried to make the point that all the scaling for
> > > > lots of processors is nonsense.  Mr Dell says it better:
> > > >
> > > >    "Eight-way (servers) are less than 1 percent of the market and
> > > >    shrinking pretty dramatically," Dell said. "If our competitors
> > > >    want to claim they're No. 1 in eight-ways, that's fine. We
> > > >    want to lead the market with two-way and four-way (processor
> > > >    machines)."
> > > >
> > > > Tell me again that it is a good idea to screw up uniprocessor
> > > > performance for 64 way machines.  Great idea, that.  Go Dinosaurs!
> > > And does your 4 way have hyperthreading?
> > What part of "shrinking pretty dramatically" did you not understand?
> > Maybe you know more than Mike Dell.  Could you share that insight?
>
> Ok. But only because you asked nicely.
>
> Mike Dell wants to sell 2 and 4 processor boxes and Intel wants to sell 
> processors with hyperthreading on them. Scaling to 4 or 8 threads is just
> like scaling to 4 or 8 processors, only worse.
>
> However, lets not end up in a yet another 64 way scalability argument here.
>
> The thing we should be worrying about is the UP -> 2 way SMP scalability
> issue. If every chip in the future has hyperthreading then all of sudden
> everyone is running an SMP kernel. And what hurts us?
>
> atomic ops
> memory barriers
>
> Ive always worried about those atomic ops that only appear in an SMP
> kernel, but Rusty recently reminded me its the same story for most of the
> memory barriers.
>
> Things like RCU can do a lot for this UP -> 2 way SMP issue. The fact it
> also helps the big end of town is just a bonus.

I think LM advocates aiming single image scalability at or before the knee
of the CPU vs performance curve. Say thats 4 way, it means you should get
good performance on 8 ways while keeping top performance on 1 and 2 and 4
ways. (Sorry if I mis-represent your position).

I don't think anyone advocates sacrificing UP performance for 32 ways, but
as he says it can happen .1% at a time.

But it looks like 2.6 will scale well to 16 way and higher. I wonder if
there are many regressions from 2.4 or 2.2 on small systems.


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