Re: Scaling noise

Larry McVoy
2003-09-03T05:09:09Z

On Wed, Sep 03, 2003 at 02:33:56PM +1000, CaT wrote:
> I think Anton is referring to the fact that on a 4-way cpu machine with
> HT enabled you basically have an 8-way smp box (with special conditions)
> and so if 4-way machines are becoming more popular, making sure that 8-way
> smp works well is a good idea.

Maybe this is a better way to get my point across. Think about more CPUs
on the same memory subsystem. I've been trying to make this scaling point
ever since I discovered how much cache misses hurt. That was about 1995
or so. At that point, memory latency was about 200 ns and processor speeds
were at about 200Mhz or 5 ns. Today, memory latency is about 130 ns and
processor speeds are about .3 ns. Processor speeds are 15 times faster and
memory is less than 2 times faster. SMP makes that ratio worse.

It's called asymptotic behavior. After a while you can look at the graph
and see that more CPUs on the same memory doesn't make sense. It hasn't
made sense for a decade, what makes anyone think that is changing?
--
---
Larry McVoy lm at bitmover.com http://www.bitmover.com/lm