Messages in this thread | | | Date | Wed, 03 Sep 2003 16:55:55 +1000 | From | Nick Piggin <> | Subject | Re: Scaling noise |
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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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