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Re: support for more than 32 CPUs

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.

> Thor


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