[libvirt] [PATCH v2] numa: warn if numa 'mem' option or default RAM splitting between nodes is used.

[Igor Mammedov]

On Wed, 6 Mar 2019 18:16:08 +0000
Daniel P. Berrangé <berrange at redhat.com> wrote:

> On Wed, Mar 06, 2019 at 06:33:25PM +0100, Igor Mammedov wrote:
> > Amend -numa option docs and print warnings if 'mem' option or default RAM
> > splitting between nodes is used. It's intended to discourage users from using
> > configuration that allows only to fake NUMA on guest side while leading
> > to reduced performance of the guest due to inability to properly configure
> > VM's RAM on the host.
> > 
> > In NUMA case, it's recommended to always explicitly configure guest RAM
> > using -numa node,memdev={backend-id} option.
> > 
> > Signed-off-by: Igor Mammedov <imammedo at redhat.com>
> > ---
> >  numa.c          |  5 +++++
> >  qemu-options.hx | 12 ++++++++----
> >  2 files changed, 13 insertions(+), 4 deletions(-)
> > 
> > diff --git a/numa.c b/numa.c
> > index 3875e1e..42838f9 100644
> > --- a/numa.c
> > +++ b/numa.c
> > @@ -121,6 +121,8 @@ static void parse_numa_node(MachineState *ms, NumaNodeOptions *node,
> >  
> >      if (node->has_mem) {
> >          numa_info[nodenr].node_mem = node->mem;
> > +        warn_report("Parameter -numa node,mem is obsolete,"
> > +                    " use -numa node,memdev instead");
> 
> My comments from v1 still apply. We must not do this as long as
> libvirt has no choice but to continue using this feature.
It has a choice to use 'memdev' whenever creating a new VM and continue using 'mem' with exiting VMs.

> 
> >      }
> >      if (node->has_memdev) {
> >          Object *o;
> > @@ -407,6 +409,9 @@ void numa_complete_configuration(MachineState *ms)
> >          if (i == nb_numa_nodes) {
> >              assert(mc->numa_auto_assign_ram);
> >              mc->numa_auto_assign_ram(mc, numa_info, nb_numa_nodes, ram_size);
> > +            warn_report("Default splitting of RAM between nodes is obsolete,"
> > +                        " Use '-numa node,memdev' to explicitly define RAM"
> > +                        " allocation per node");
> >          }
> >  
> >          numa_total = 0;
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 1cf9aac..61035cb 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -206,10 +206,14 @@ For example:
> >  -numa cpu,node-id=0,socket-id=0 -numa cpu,node-id=1,socket-id=1
> >  @end example
> >  
> > - at samp{mem} assigns a given RAM amount to a node. @samp{memdev}
> > -assigns RAM from a given memory backend device to a node. If
> > - at samp{mem} and @samp{memdev} are omitted in all nodes, RAM is
> > -split equally between them.
> > + at samp{memdev} assigns RAM from a given memory backend device to a node.
> > +
> > +Legacy options/behaviour: @samp{mem} assigns a given RAM amount to a node.
> > +If @samp{mem} and @samp{memdev} are omitted in all nodes, RAM is split equally
> > +between them. Option @samp{mem} and default RAM splitting are obsolete as they
> > +do not provide means to manage RAM on the host side and only allow QEMU to fake
> > +NUMA support which in practice could degrade VM performance.
> > +It's advised to always explicitly configure NUMA RAM by using the @samp{memdev} option.
> >  
> >  @samp{mem} and @samp{memdev} are mutually exclusive. Furthermore,
> >  if one node uses @samp{memdev}, all of them have to use it.
> > -- 
> > 2.7.4
> > 
> 
> Regards,
> Daniel