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SubjectRe: Threads FAQ entry incomplete
"J.D. Bakker" wrote:
>
> At 13:42 -0600 20-06-2001, Charles Cazabon wrote:
> >Rodrigo Ventura <yoda@isr.ist.utl.pt> wrote:
> > > BTW, I have a question: Can the availability of dual-CPU boards for intel
> >> and amd processors, rather then tri- or quadra-CPU boards, be explained with
> >> the fact that the performance degrades significantly for three or more CPUs?
> >> Or is there a technological and/or comercial reason behind?
> >
> >Commercial reasons. Cost per motherboard/chipset goes way up as the number of
> >CPUs supported goes up. For each CPU that a chipset supports, it has to add a
> >lot of pins/lands, and chipsets are already typically land-limited.
>
> That's not quite accurate. Most modern SMP-able processors have a
> common bus, where going from 1->2 CPUs adds just a handful of extra
> nets (usually bus request, bus grant and some IRQs). The actual
> issues are threefold.

Some SMP chipset/cpu combos allow direct cache-to-cache update when a
dirty cache line is found through snooping, while the lower performance
ones don't. Wouldn't any kind of cache-to-cache direct update that
bypasses the main bus also add physical complexity (extra traces)? And
wouldn't that become more important as the number of cpu's goes up?

>
> First, most commodity chipsets simply support no more than two CPUs
> at best; most CPUs don't support having more (or any) siblings.
> Adding more is cheap on the ASIC level, but nobody bothers because
> there is no demand.
>
> Second, adding more CPUs on a shared bus decreases the bus bandwidth
> that is available per CPU. This is comparable with having Ethernet
> hubs vs switches. The really expensive multi-CPU boards have crossbar
> switches between CPUs, memory and PCI. Future stuff like RapidIO may
> mitigate this.
>
> Third, the more CPUs a bus holds, the higher the capacitance on the
> bus lines. Higher capacitance means lower maximum bus speed, which
> aggravates point two.
>
> >Motherboard trace complexity (and therefore number of layers) goes up. Add to
> >that that the potential market goes down as CPUs goes up.
>
> True enough.
>
> Regards,
>
> JDB
> [working on a SMP PowerPC design]
> --
> LART. 250 MIPS under one Watt. Free hardware design files.
> http://www.lart.tudelft.nl/
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