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

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

[Greg Kurz]

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

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
---

This was tested with a pseries guest only. I'd like to know if it is okay
for other platforms.

 include/sysemu/sysemu.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
index 38fb3cad35e1..89d742caa477 100644
--- a/include/sysemu/sysemu.h
+++ b/include/sysemu/sysemu.h
@@ -185,7 +185,7 @@ extern int mem_prealloc;
  *
  * Note that cpu->get_arch_id() may be larger than MAX_CPUMASK_BITS.
  */
-#define MAX_CPUMASK_BITS 255
+#define MAX_CPUMASK_BITS 1024
 
 #define MAX_OPTION_ROMS 16
 typedef struct QEMUOptionRom {

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

[Igor Mammedov]

On Wed, 11 May 2016 16:00:04 +0200
Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:

> 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
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
> 
> This was tested with a pseries guest only. I'd like to know if it is okay
> for other platforms.
#git grep MAX_CPUMASK_BITS
...
hw/arm/virt.c:    mc->max_cpus = MAX_CPUMASK_BITS;
hw/ppc/spapr.c:    mc->max_cpus = MAX_CPUMASK_BITS;
...

you probably don't want to change all those to 1024,
see/review a similar patch where I bump it upto 288 bits
https://lists.gnu.org/archive/html/qemu-devel/2016-05/msg01101.html


in addition where does 1024 come from?
is pseries capable to provide that many CPUs?

> 
>  include/sysemu/sysemu.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> index 38fb3cad35e1..89d742caa477 100644
> --- a/include/sysemu/sysemu.h
> +++ b/include/sysemu/sysemu.h
> @@ -185,7 +185,7 @@ extern int mem_prealloc;
>   *
>   * Note that cpu->get_arch_id() may be larger than MAX_CPUMASK_BITS.
>   */
> -#define MAX_CPUMASK_BITS 255
> +#define MAX_CPUMASK_BITS 1024
>  
>  #define MAX_OPTION_ROMS 16
>  typedef struct QEMUOptionRom {
> 

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

[Greg Kurz]

On Wed, 11 May 2016 16:20:08 +0200
Igor Mammedov <imammedo@redhat.com> wrote:
> On Wed, 11 May 2016 16:00:04 +0200
> Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> 
> > 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
> > 
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> > 
> > This was tested with a pseries guest only. I'd like to know if it is okay
> > for other platforms.  
> #git grep MAX_CPUMASK_BITS
> ...
> hw/arm/virt.c:    mc->max_cpus = MAX_CPUMASK_BITS;
> hw/ppc/spapr.c:    mc->max_cpus = MAX_CPUMASK_BITS;
> ...
> 
> you probably don't want to change all those to 1024,

Well, at least the spapr one :)

> see/review a similar patch where I bump it upto 288 bits
> https://lists.gnu.org/archive/html/qemu-devel/2016-05/msg01101.html
> 

Indeed... I should set it to 255 in hw/arm/virt.c. Thanks for pointing
that out.

> 
> in addition where does 1024 come from?

All PowerPC distros I could check support either 1024 (PowerKVM,
Fedora) or 2048 (Ubuntu, RHEL7) vCPUs in KVM.

I've chosen 1024 because  of PowerKVM :)

> is pseries capable to provide that many CPUs?
> 

Yes.

> > 
> >  include/sysemu/sysemu.h |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> > index 38fb3cad35e1..89d742caa477 100644
> > --- a/include/sysemu/sysemu.h
> > +++ b/include/sysemu/sysemu.h
> > @@ -185,7 +185,7 @@ extern int mem_prealloc;
> >   *
> >   * Note that cpu->get_arch_id() may be larger than MAX_CPUMASK_BITS.
> >   */
> > -#define MAX_CPUMASK_BITS 255
> > +#define MAX_CPUMASK_BITS 1024
> >  
> >  #define MAX_OPTION_ROMS 16
> >  typedef struct QEMUOptionRom {
> >   
>