Re: support for more than 32 CPUs

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Mon, Apr 16, 2012 at 10:54:44PM +0100, Mindaugas Rasiukevicius wrote:
> Thor Lancelot Simon <tls%panix.com@localhost> wrote:
> > On Mon, Apr 16, 2012 at 11:05:04PM +0200, Manuel Bouyer wrote:
> > > Hello,
> > > the attached patch, based on a patch sent by Mindaugas Rasiukevicius
> > > on tech-kern@ some time ago, bumps the max number of CPUs to 256 for
> > > amd64, and should easily allow up to 64 for Xen/amd64. I tested it
> > > on a x86 with 64 AMD cores (lighly as this box has now known drive yet
> > > - some driver hacking is needed), 8 intel cores and with a Xen domU
> > > with 4 core. I didn't notice regressions so far.
> > > 
> > > Comments before I commit ?
> > 
> > Only that for even a compilation workload, we seem to lose performance
> > after about 15 CPUs, so though we'll probe more CPUs this way... there
> > may be less benefit than one would expect.
> 
> It depends on the workload.  The ~16 CPUs threshold you are talking about
> applies when there is a high pressure on UVM and global page queue locks
> get contended.  For some other workloads, we scale better.
> 
> In any case, this is a step to the right direction.

It was not clear in my original email, but without this patch NetBSD
doesn't boot on such a machine. Even if performances are not there yet,
at last we can run benchmarks now :)
Or we can turn it into a Xen dom0 ...

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--