Re: Scaling noise

[Nick Piggin <piggin@cyberone.com.au>]

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.