Re: [Qemu-devel] [Qemu-ppc] [PATCH qemu] sysemu: support up to 1024 vCPUs

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Feb 24, 2017 at 10:13:50AM +0100, Greg Kurz wrote:
> On Fri, 24 Feb 2017 15:55:31 +1100
> Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> 
> > From: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > 
> > Some systems can already provide more than 255 hardware threads.
> > 
> > Bumping the QEMU limit to 1024 seems reasonable:
> > - it has no visible overhead in top;
> > - the limit itself has no effect on hot paths.
> > 
> > Cc: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> > ---
> > 
> > With ulimit -u/-n bumped (nproc and nofile), I was able to boot a guest
> > with 1024 CPUs, both with threads=1 and threads=8.
> > 
> > It takes time though - 3:15 to get to the guest shell but it is probably
> > expected on 160-threads machine.

Yes, I'd expect so, that's a lot of overcommit.  Plus, switching from
one vcpu to another on the same host thread will, IIRC, require two
full partition switches, which are pretty slow on Power.

> I remember something similiar at the time... also I had to give more
> RAM to the guest to be able to run 1024 CPUs (sth like 6 gigs versus
> 512 megs for 1 CPU). With the same amount of guest RAM, each extra CPU
> would cause the memory used by QEMU to grow about 8 megs.

Hm... that seems like rather a lot.  Any idea why?

> 
> > ---
> >  hw/ppc/spapr.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index e465d7ac98..46b81a625d 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -2712,7 +2712,7 @@ static void spapr_machine_class_init(ObjectClass *oc, void *data)
> >      mc->init = ppc_spapr_init;
> >      mc->reset = ppc_spapr_reset;
> >      mc->block_default_type = IF_SCSI;
> > -    mc->max_cpus = 255;
> > +    mc->max_cpus = 1024;
> >      mc->no_parallel = 1;
> >      mc->default_boot_order = "";
> >      mc->default_ram_size = 512 * M_BYTE;
> 
> 



-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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