Re: How many pentium-3 processors does SMP support?

[Ralf Baechle <ralf@uni-koblenz.de>]

On Wed, Jul 11, 2001 at 02:25:09AM +1200, Chris Wedgwood wrote:

>     The number of CPUs is currently globally limited to 32 by NR_CPUS in
>     include/linux/threads.h.
> 
> Really?
> 
> <pause>

A define which can easily be changed.

> 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.

The next limit are bitfields for processors in a number of places.  They're
stored in unsigned long variables, so this limits the kernel to 32
processors on 32-bit machines and 64 on 64-bit machines.  Kanoj once
fixed that and we had Linux booting on a 128p Origin but I'm not sure
if those fixes went back to Linus.  Once this is fixed there is a number
of arrays with NR_CPUS elements in struct task_struct.  Once you reach
an estimated number of 200-300 processors (for 16kb kernel stacks) those
make task_struct that large that the kernel stack may overflow.  I
haven't really researched all the gotchas that may cause problems with
the kernel when going to machines of more than 128 processors.

  Ralf