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

[Greg Kurz <groug@kaod.org>]

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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.
> 

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.

> ---
>  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;



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