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SubjectRe: How many pentium-3 processors does SMP support?
Chris Wedgwood wrote:
> On Tue, Jul 10, 2001 at 04:19:43PM +0200, Christoph Hellwig wrote:
> The number of CPUs is currently globally limited to 32 by NR_CPUS in
> include/linux/threads.h.
> Really?
> <pause>
> Ah, so it is... yes, making this architecture dependant might be a
> good idea. Large PPC and MIPS boxen need to adjust this already. Also,
> someone did a starfire port, I think that had 64 processors, not sure.
> You can. But you cannot buy 32-processor PII (-Xeon) systems that are
> supported by Linux.
> What is the limit here? The 8/16 way SE chipsets?
> > In anyone from Compaq is reading this, you should send me a 32-way
> > Xeon ASAP just to prove they really work :)
> It doesn't.
> Oh, then they definately need to send me one.
> Are these not MP1.4 based? Something different?
> --cw

For IA32/i386/x86:

The Linus-kernel only supports a maximum of 15 Pentium IIIs
due to APIC addressing (4 bits, with 0xf meaning "broadcast").

Pentium 4 uses 0xff for broadcast, so lots more of them can
be supported (when you can find a P-4 MP server).

I have heard of some IBM/Sequent patches that modify the
logical vs. physical APIC addressing scheme to make 16-way
systems work.

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