Messages in this thread | | | Date | 4 Jan 2005 12:09:40 +0100 | Date | Tue, 4 Jan 2005 12:09:40 +0100 | From | Andi Kleen <> | Subject | Re: Max CPUs on x86_64 under 2.6.x |
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On Mon, Jan 03, 2005 at 06:20:34PM -0800, William Lee Irwin III wrote: > "Colin Coe" <colin@coesta.com> writes: > >> Why is the number of CPUs on the x86_64 architecture only 8 but under i386 > >> it is 255? > >> I've searched the list archives and Google but can't find an answer. > > On Tue, Jan 04, 2005 at 01:34:50AM +0100, Andi Kleen wrote: > > Post 2.6.10 x86-64 will support more CPUs. 2.6.10 actually does too, > > but the Kconfig hadn't been changed then. Previously there was an > > 8 CPU APIC driver limit, however it turned out later that it doesn't > > apply to some Opteron machines. > > Barring cpus with a different onboard interrupt controller from the > xAPIC or the use of external interrupt controllers to assist with cpu > addressing, 255 serves as an architectural limit for Opteron as well.
Yes, 255 is the limit, but not 8. Opteron can enable a special flat mode that allows flat APIC addressing upto 255. I assume the BIOS will set that bit on machines with that many CPUs.
I recently audited the flat APIC code and I think it should work without changes to 255 CPUs, but I wasn't able to test it so far.
However 2.6.10 supports clustered mode now anyways, so you could probably use more CPUs given the right x86-64 IBM machine. I don't know if that has been tested so far.
The change to extend NR_CPUs and the max number of nodes just went into Linus' tree, you would need a recent bk snapshot.
http://linux.bkbits.net:8080/linux-2.6/cset@41da1ff62QYI89HDgrcKwnBAz6wgQg?nav=index.html|ChangeSet@-1d
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