On Fri, Feb 24, 2017 at 10:13:50AM +0100, Greg Kurz wrote: > On Fri, 24 Feb 2017 15:55:31 +1100 > Alexey Kardashevskiy <a...@ozlabs.ru> wrote: > > > From: Greg Kurz <gk...@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 <gk...@linux.vnet.ibm.com> > > Signed-off-by: Alexey Kardashevskiy <a...@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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