J.D. Bakker <[EMAIL PROTECTED]> wrote:
> At 13:42 -0600 20-06-2001, Charles Cazabon wrote:
> >Rodrigo Ventura <[EMAIL PROTECTED]> 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.
Low-end Intel multi-CPU chipsets are like this (typical 2-CPU configurations,
and low-end 4-CPU configurations). Higher-end systems (8-way, etc) typically
have multiple processor busses, with only one, two, or four processors per bus.
Processor bus contention costs performance even in 2-way systems, and at 4-way
and above, it becomes a serious bottleneck. High end chipsets do the cache
coherency and snooping control between the busses. Other N-way chipsets
(i.e., non-Intel) have point-to-point links between each CPU and the chipset.
The new AMD 760 chipset for Athlon is like this; so are N-way Alpha chipsets.
I can't swear to other hardware.
> 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.
Ask ServerWorks about this. They make 16-way Intel chipsets. It's possible,
just not cheap.
> 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.
Which is one of the reasons for a pont-to-point "bus" with Alpha and Athlon
CPUs.
Charles
--
-----------------------------------------------------------------------
Charles Cazabon <[EMAIL PROTECTED]>
GPL'ed software available at: http://www.qcc.sk.ca/~charlesc/software/
My opinions are just that -- my opinions.
-----------------------------------------------------------------------
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
